Add support for a data-video attribute on image thumbnails for YouTube videos.

* If they contain valid YouTube URL's these will be replaced on click with the embed code.
* The embedded iframe for YouTube is also resized properly, and scaled to fit between caption and toolbar.
 AFAIK It's not possible to get the toolbars floating over the embedded video with z-indexes, etc.
* Added a setting for disabling the hiding of caption and toolbar on tap. This is nice if you use videos,
 as they steal focus, the user experience can become a little cumbersome.