Adding ':' as a supported character in Macros.

Summary: Adding ':' in order to support SA-style smiley conventions (e.g: :allears:) in Phabricator.

Test Plan: Tested working on local Phabricator copy.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D4727