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