Restore ANSI highlighting of lint sections to lint output

Summary: Fixes T9846. This restores the last missing feature, ANSI highlighting of diff sections.

Test Plan:
Added a mode so we can actually test this stuff, activated that mode, wrote unit tests.

Did a bunch of actual lint locally too and looked at it, all seemed sane.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9846

Differential Revision: https://secure.phabricator.com/D18512