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