Add support for Media Extension

[chrisppy]

The specification is located here

[azmeuk]

For the record, Media-RSS is used by youtube in their feeds.

[thedanhoffman]

I would like this as well, or at least the option to specify arbitrary strings

[Qwertie-]

This would be nice. I had a quick look in it and its quite a chunk of work to get done though as the spec includes a whole lot of flexibility and annoying logic with multiple ways to specify metadata.