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