Add --quiet flags to the ArcanistCSSLintLinter.

Summary: This isn't really necessary, nor does it bear any significant benefits, however semantically it seems to make sense. From csslint --help, the --quiet flag causes csslint to "Only output when errors are present".

Test Plan:
Tested using csslint directly:

> csslint test.css

csslint: No errors in /home/joshua/workspace/github.com/facebook/arcanist/foo.css.
> csslint --quiet test.css

Reviewers: chad, epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9036