Add WARNING and IMPORTANT to remarkup

Summary:
Ref T3116. This is basically the NOTE: rule except we consume WARNING and IMPORTANT.

Also adds (X) syntax for all three of (NOTE), (WARNING), and (IMPORTANT), which behave like the colon versions, just eating the keywords in html rendering.

Test Plan: played with it in phabricator proper; see next diff. also wrote a unit test

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T3116

Differential Revision: https://secure.phabricator.com/D8052