Add "wide" remarkup image support for Documents

Summary: Seems to work ok, if you give size=wide to an image file, we blow it out a bit in DocumentPro mode.

Test Plan:
Test in Phame and Maniphest.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15840