Add --purge-user to bin/cache purge

Summary: Ref T4103. Provide a CLI mechanism for purging the user cache.

Test Plan:

• Purged with --purge-user and --purge-all.
• Verified cache table got wiped.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4103

Differential Revision: https://secure.phabricator.com/D16033