Add more options to Remarkup
Summary:
See D1416. Add options to file-embed syntax, and document new code and
embed options.
Test Plan: Used new options in markup blocks.
Reviewers: davidreuss, btrahan, jungejason
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T336
Differential Revision: https://secure.phabricator.com/D1417