arc unit --output none

Summary:
Support for no output from arc unit (For scripts, etc).
Also include --output param, analogous to arc lint --output.

Test Plan: run all 6 variants + `--output bad-value`

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D6642