Create a "Capabilities" plugin

This is based on an IM conversation I had with Jesse MacFadyen. I promised to write it up. HIs idea, my write up, so any mistakes blame on me.

I recently logged an ER for supporting a "IsCameraAvailable" request in the Camera API. JF mentioned that a general purpose Capabilities API would be a good place for this. Specifically a CameraCapabilities plugin.

You could then have a MediaCapabilities plugin to support things like canPlayMp3, canPlayWav, etc.

Basically - one plugin per 'area' of capabilities.