PHPCS supports reading from stdin.

Summary: PHPCS does actually support reading data from stdin, so let's make ArcanistExternalLinter aware of this.

Test Plan: arc unit.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9034