Add ability to pass width arg to dot remarkup interpeter

Summary:
Allows to keep really wide graphs inside the div:

dot (width=100%) {{{ }}}

Test Plan:
Created a graph that is wider than a phriction page.
Added (width=100%) and now the images stays within the div.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8235