Let PlayerContextGroup handle media outside of Cassette

[benwiley4000]

It's possible we'll have media on the page other than normally embedded media elements, like a YouTube embed, which we'll want to play nice with other media on the page. We can add some new context props to a PlayerContextGroup consumer that would allow hooking in a custom media thing. For example, we would want to make it work with the YouTube Iframe Player API. https://developers.google.com/youtube/iframe_api_reference

The trouble is there doesn't seem to be a simple way to listen to volume changes, so we'll have to think about that...