Version 2.0.0-beta.1

First beta release of Pico's second major release. This is a pre-release!

* [New] Pico is on its way to its second major release!
* [New] Improve Pico's release & build process
* [New] Add "Developer Certificate of Origin" to `CONTRIBUTING.md`
* [New] Add license & copyright header to all relevant files
* [New] Add Pico version constants (`Pico::VERSION` and `Pico::VERSION_ID`),
        and add a `version` Twig variable and `%version%` Markdown placeholder
* [New] Add Pico API versioning for plugins (see `Pico::API_VERSION` constant);
        Pico now triggers events on plugins using the latest API version only
        ("native" plugins), `PicoDeprecated` takes care of all other plugins;
        as a result, old plugin's always depend on `PicoDeprecated` now
* [New] Add a theme and plugin installer for composer; Pico now additionally
        uses the new `vendor/pico-plugin.php` file to discover plugins
        installed by composer and loads them using composer's autoloader;
        see the `picocms/composer-installer` repo for more details; Pico
        loads plugins installed by composer first and ignores conflicting
        plugins in Pico's `plugins/` dir
* [New] Add `$enableLocalPlugins` parameter to `Pico::__construct()` to allow
        website developers to disable local plugin discovery by scanning the
        `plugins/` dir (i.e. load plugins from `vendor/pico-plugin.php` only)
* [New] Add public `AbstractPicoPlugin::getPluginConfig()` method
* [New] Add public `Pico::loadPlugin()` method and the corresponding
        `onPluginManuallyLoaded` event
* [New] Add public `Pico::resolveFilePath()` method (replaces the protected
        `Pico::discoverRequestFile()` method)
* [New] Add public `Pico::is404Content()` method
* [New] Add public `Pico::getYamlParser()` method and the corresponding
        `onYamlParserRegistered` event
* [New] Add public `Pico::substituteFileContent()` method
* [New] Add public `Pico::getPageId()` method
* [New] Add public `Pico::getFilesGlob()` method
* [New] Add public `Pico::getVendorDir()` method, returning Pico's installation
        directory (i.e. `/var/www/pico/vendor/picocms/pico`); don't confuse
        this with composer's `vendor/` dir!
* [New] Add `$default` parameter to `Pico::getConfig()` method
* [New] Add empty `assets/` and `content/` dirs
* [New] #305: Add `url_param` and `form_param` Twig functions, and the public
        `Pico::getUrlParameter()` and `Pico::getFormParameter()` methods,
        allowing theme developers to access URL GET and HTTP POST parameters
* [New] Add `$meta` parameter to `markdown` Twig filter
* [New] Add `remove` fallback to `sort_by` Twig filter
* [New] Add `theme_url` config parameter
* [New] Add public `Pico::getBaseThemeUrl()` method
* [New] Add `REQUEST_URI` routing method, allowing one to simply rewrite all
        requests to `index.php` (e.g. use `FallbackResource` or `mod_rewrite`
        in your `.htaccess` for Apache, or use `try_files` for nginx)
* [New] #299: Add built-in 404 page as fallback when no `404.md` is found
* [New] Allow sorting pages by arbitrary meta values
* [New] Add `onSinglePageLoading` event, allowing one to skip a page
* [New] Add `onSinglePageContent` event
* [New] Add some config parameters to change Parsedown's behavior
* [Changed] ! Disallow running the same Pico instance multiple times by
            throwing a `RuntimeException` when calling `Pico::run()`
* [Changed] ! #203: Load plugins from `plugins/<plugin name>/<plugin name>.php`
            and `plugins/<plugin name>.php` only (directory and file name must
            match case-sensitive), and throw a `RuntimeException` when Pico is
            unable to load a plugin; also throw a `RuntimeException` when
            superfluous files or directories in `plugins/` are found; use a
            scope-isolated `require()` to include plugin files
* [Changed] ! Use a plugin dependency topology to sort `Pico::$plugins`,
            changing the execution order of plugins so that plugins, on which
            other plugins depend, are always executed before their dependants
* [Changed] ! Don't pass `$plugins` parameter to `onPluginsLoaded` event by
            reference anymore; use `Pico::loadPlugin()` instead
* [Changed] ! Leave `Pico::$pages` unsorted when a unknown sort method was
            configured; this usually means that a plugin wants to sort it
* [Changed] Overhaul page discovery events: add `onPagesDiscovered` event which
            is triggered right before `Pico::$pages` is sorted and move the
            `$currentPage`, `$previousPage` and `$nextPage` parameters of the
            `onPagesLoaded` event to the new `onCurrentPageDiscovered` event
* [Changed] Move the `$twig` parameter of the `onPageRendering` event to the
            `onTwigRegistered` event, replacing the `onTwigRegistration` event
* [Changed] Unify the `onParsedownRegistration` event by renaming it to
            `onParsedownRegistered` and add the `$parsedown` parameter
* [Changed] #330: Replace `config/config.php` by a modular YAML-based approach;
            you can now use a arbitrary number of `config/*.yml` files to
            configure Pico
* [Changed] ! When trying to auto-detect Pico's `content` dir, Pico no longer
            searches just for a (possibly empty) directory, but rather checks
            whether a `index.md` exists in this directory
* [Changed] ! Use the relative path between `index.php` and `Pico::$themesDir`
            for Pico's theme URL (also refer to the new `theme_url` config and
            the public `Pico::getBaseThemeUrl()` method for more details)
* [Changed] #347: Drop the superfluous trailing "/index" from Pico's URLs
* [Changed] Flip registered meta headers array, so that the array key is used
            to search for a meta value and the array value is used to store the
            found meta value (previously it was the other way round)
* [Changed] ! Add lazy loading for `Pico::$yamlParser`, `Pico::$parsedown` and
            `Pico::$twig`; the corresponding events are no longer part of
            Pico's event flow and are triggered on demand
* [Changed] ! Trigger the `onMetaHeaders` event just once; the event is no
            longer part of Pico's event flow and is triggered on demand
* [Changed] Don't lower meta headers on the first level of a page's meta data
            (i.e. `SomeKey: value` is accessible using `$meta['SomeKey']`)
* [Changed] Don't compare registered meta headers case-insensitive, require
            matching case
* [Changed] Allow users to explicitly set values for the `date_formatted` and
            `time` meta headers in a page's YAML front matter
* [Changed] Add page siblings for all pages
* [Changed] ! Treat pages or directories that are prefixed by `_` as hidden;
            when requesting a hidden page, Pico responds with a 404 page;
            hidden pages are still in `Pico::$pages`, but are moved to the end
            of the pages array when sorted alphabetically or by date
* [Changed] ! Don't treat explicit requests to a 404 page as successful request
* [Changed] Change method visibility of `Pico::getFiles()` to public
* [Changed] Change method visibility of `Pico::triggerEvent()` to public;
            at first glance this method triggers events on native plugins only,
            however, `PicoDeprecated` takes care of triggering events for other
            plugins, thus you can use this method to trigger (custom) events on
            all plugins; never use it to trigger Pico core events!
* [Changed] Move Pico's default theme to the new `picocms/pico-theme` repo; the
            theme was completely rewritten from scratch and is a much better
            starting point for creating your own theme; refer to the theme's
            `CHANGELOG.md` for more details
* [Changed] Move `PicoDeprecated` plugin to the new `picocms/pico-deprecated`
            repo; refer to the plugin's `CHANGELOG.md` for more details
* [Changed] Update dependencies: Twig 1.34, Symfony YAML 2.8, Parsedown 1.6
* [Changed] Improve Pico docs and PHP class docs
* [Changed] A vast number of small improvements and changes...
* [Removed] ! Remove `PicoParsePagesContent` plugin
* [Removed] ! Remove `PicoExcerpt` plugin
* [Removed] Remove `rewrite_url` and `is_front_page` Twig variables
* [Removed] Remove superfluous parameters of various events to reduce Pico's
            error-proneness (plugins hopefully interfere with each other less)