HTML emails + basic DIFF colorizing

Summary:
Added support for side-by-side HTML and plaintext email building.
Also generate slightly colorized diff content in emails.

This is enabled by a new config, metamta.html-emails

Test Plan: Tested locally, but needs some unit tests probably.

Reviewers: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9375