Add a rainbow syntax highlighter
Summary: See https://www.facebook.com/epriestley/posts/969735599979
Test Plan:
https://secure.phabricator.com/file/view/PHID-FILE-b4def7487b2c35295a19/
Reviewed By: codeblock
Reviewers: tristanfisher, jungejason, tuomaspelkonen, aran, codeblock
CC: aran, epriestley, codeblock
Differential Revision: 768