Closes #2.
Closes #5.

Major changes so far (See the changelog for more):

• The info, readme, releases, files, owner, and owned commands now take exactly one positional argument, and the output formats of many of these commands have been simplified for this case.
• Package yanking is now honored.
• There is now a library API.
• Package arguments to info, files, and readme can now be requirements specifiers (e.g., click>=5.0, <8.1) that are used to filter the available versions

To do:

• Add a note to the README about search being disabled
• Add public documentation for the new library API
  • Set up a Read the Docs site?
• Update all docs (including command docstrings and command example output)
• Add QyPIError subclasses for missing projects vs. no matching versions
• Add an option (--no-pre?) for erroring rather than falling back to prereleases when a project only has prereleases
• More tests
  • Test --yanked/--no-yanked options
  • Test QyPI & related classes directly without involving the click interface
• Account for the fact that the version-specific endpoints no longer include a releases key in their responses (ref)
• Try to make the use of "release" vs. "version" consistent
  • Give releases a versions synonym? (Use click-aliases?)
  • Rename releases to versions?
• Give --latest-version a shorter alternative spelling