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