Allow appending to safe HTML

Summary:
Sometimes I need to return safe HTML and I ended up calling `hsprintf('%s%s', $tag1, $tag2)`.
This looks better.

Test Plan: New unit test.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4765