Handle `arc --version`.

Summary: It seems reasonable to transform `arc --version` into `arc version`. The version command is similar to the help command in that it is a command that users unfamiliar with Arcanist will probably try to run at some stage.

Test Plan: Ran `arc --version`.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9274