Modularize "bin/cache" purgers

Summary: Ref T12859. This is an older command with a lot of hard-coded flags. Modularize cache purging in a modern way so it can be extended.

Test Plan: Ran bin/cache purge --trace with various valid and invalid flags.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12859

Differential Revision: https://secure.phabricator.com/D18146