Add YouTube embedding

Hooks into the existing image embed system with its show/hide.

Embed using the Markup image embed syntax:
Eg. ![](http://www.youtube.com/watch?v=NFZY_o0QoJk)

Supports only youtube links of the form:
youtube.com/watch?v=[embed_id]
Merge pull request #93 from stevetan/master

Add YouTube embedding