Add a LESS linter.

Summary: This class provides an adapter for [[https://github.com/less/less.js/ | lessc]].

Test Plan: Wrote and executed unit tests.

Reviewers: chad, #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8992