Allow Remarkup headers to generate a TOC

Summary: Allow header rules to generate with anchors and build a table of
contents.

Test Plan:
https://secure.phabricator.com/file/view/PHID-FILE-lttr6jszx2y2rqjpalr6/

Reviewers: btrahan, jungejason, davidreuss

Reviewed By: btrahan

CC: aran, btrahan

Differential Revision: https://secure.phabricator.com/D1334