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