Add UI to use a YouTube video

You now have the choice between videos in your media library (or external) and YouTube videos.
![Screen Shot 2013-03-20 at 18 05 36 ](https://f.cloud.github.com/assets/1168/281824/e6aa7468-9180-11e2-964b-b7ad55477998.png)

If you choose YouTube, you can either enter the YouTube video ID or the full YouTube URL (as in the [Video Publishing Assistant](http://docs.sublimevideo.net/video-publishing-assistant)).
![Screen Shot 2013-03-20 at 18 05 55 ](https://f.cloud.github.com/assets/1168/281825/e9f5e65c-9180-11e2-914a-89510b1203ce.png)

The live preview then displays the YouTube video.
![Screen Shot 2013-03-20 at 18 06 17 ](https://f.cloud.github.com/assets/1168/281826/ec2d69b8-9180-11e2-9ade-536690ee1b5d.png)

This is the generated shortcode once you click "Add to my post".
![Screen Shot 2013-03-20 at 18 07 04 ](https://f.cloud.github.com/assets/1168/281827/f1722864-9180-11e2-8d27-1da265fbd954.png)

Feedback welcome!