Add isAudioOnly to player interface

[avelad]

Is it possible to add an isAudioOnly to player interface, in the same way as isInProgress eg.?

Note: in the demo app, the form for get this is not very clear: https://github.com/google/shaka-player/blob/master/demo/asset_section.js#L211-L219

[joeyparrish]

Good idea! This should be easy to do.