Support YouTube videos with time offsets (fixes #929)

Embed code taken from <http://stackoverflow.com/questions/13119791/can-the-ipython-notebook-youtubevideo-class-play-from-time-offset>.
Switch to using a regex to parse timestamps
Add tests for Match.youtube_videos_formatted
Added support for a few other types of YouTube timestamps

* timestamps with hours: "1h2m3s"
* raw seconds with no delimiters: "12345"

Also added a test to ensure output of videos with
no timestamp is unaffected.
Merge pull request #949 from legoktm/929

Support YouTube videos with time offsets (fixes #929)