Version 2.0.0-beta.3

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

* [Changed] Add `README.md`, `CONTRIBUTING.md` and `CHANGELOG.md` of main repo
            to pre-bundled releases, keep `.gitignore`
* [Changed] Deny access to a possibly existing `composer.phar` in `.htaccess`
* [Changed] Disallow the use of the `callback` filter for the `url_param` and
            `form_param` Twig functions
* [Changed] Improve documentation
* [Fixed] Fix page tree when sorting pages by arbitrary values
* [Fixed] Fix sorting of `Pico::$nativePlugins`