Image macros for Phabricator!

Summary:
Added long waited image macro support for differential and others.

Test Plan:
Tried a couple of different macros and made sure they appear nicely
in the comment preview. Made sure that the normal comments are shown
correctly.

Reviewed By: epriestley
Reviewers: epriestley
CC: jungejason, tuomaspelkonen, epriestley
Differential Revision: 129