Support `arc lint --output none`

Summary:
I want to run lint on background and I'm interested only in side effect of caching (and maybe exit status).
This is better than discarding stdout later because we don't do unnecessary work and error conditions are still printed.

Test Plan:
  $ arc lint --output none # with error
  $ echo $?
  $ arc lint --output none # with no lintable paths
  $ arc lint --output none # witout errors

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5106