Add <table> syntax to Remarkup

Summary:
Adds basic table syntax. I went with just HTML table syntax since it's straightforward and familiar and all the other syntaxes (in mediawiki, trac, etc) seem awful.

It's safe to do this since we parse the talbe markup itself very strictly, and don't actually use the tags the user types.

Test Plan: {F13683}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D2902