Add Highlight Remarkup Rule

Summary: Ref T5560, Adds a basic highlight rule.

Test Plan: Edit a Document and add a highlighted word.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: tycho.tatitscheff, Korvin

Maniphest Tasks: T5560

Differential Revision: https://secure.phabricator.com/D14384