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