Make countdowns (internally) embeddable

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