Make PhabricatorLintEngine extensible

Summary:
It makes sense for Phabricator customizations to use all the Phabricator linters and maybe add some more or delete some.
Doing this as proxy would be PITA as there are lots of methods that would need to be proxied.

Test Plan: Extended it.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4954