Separate Phutil specific lint rules from XHPAST

Summary: Also provides an example how to build custom linter using XHPAST.

Test Plan: Added debug output to `willLintPaths()`, verified that each path is parsed only once.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4718