Adding loop support for audio/video

Firefox doesn't support native looping as of 4.0b7. Though it sees the `loop` attr, it'll return `undefined` for its value.

To be honest I'm not sure if `Modernizr.audio/video.loop` is the appropriate place for this boolean. The only feature detection currently done is in regard to codec support which I've piggybacked on, but now `Modernizr.audio/video` stores "codecs & other stuff" which conceptually might be suboptimal.