Add simple table Remarkup rule
Summary:
Remarkup rules should have these features:
- Easy to write by hand.
- Easy to read in plain text.
- Spellchecker friendly.
Current table rule has none of it.
Test Plan: New test, looked at rendered output in Phabricator.
Reviewers: epriestley
Reviewed By: epriestley
CC: btrahan, aran, Korvin
Differential Revision: https://secure.phabricator.com/D3725