Add a version workflow to arcanist.

Summary: A version workflow would be useful, especially for less technical users. Additionally, whenever I am faced with a new command I reasonably expect $CMD [--help|help] and $CMD [--version|version] to work.

Test Plan: Ran arc version.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9265