Summary:
You can now embed countdowns in Remarkup! Not sure what it's
useful for, but there you have it.
Also I may have made a hash of the markup code; I don't really know what
I'm doing.
Test Plan: Make a new countdown, put `{C###}` in a Differential comment.
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin
Maniphest Tasks: T1053
Differential Revision: https://secure.phabricator.com/D3290
