Support audio files with HTML5 <audio />

Summary: Ref T3887. Similar to how we render images with <img />, render audio with <audio /> if possible.

Test Plan: See screenshots.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3887

Differential Revision: https://secure.phabricator.com/D7156