Remarkup allows to point a specific image in a Mock set

Summary: One can point to spesific image in a Mock set using {Mxx|yy} syntax

Test Plan: {F35373}

Reviewers: chad, epriestley

Reviewed By: epriestley

CC: aran, Korvin, AnhNhan

Maniphest Tasks: T2710

Differential Revision: https://secure.phabricator.com/D5322