Add support for audio/video attributes

- adds `currentTime` property for controlling position in a video or audio element
- adds `volume` property for controlling volume in a video or audio element
- adds `paused` mutation method for controlling play/pause state in a video or audio element
- adds `playbackRate` property for controlling playback speed in a video or audio element
- adds `srcObject` property for setting a WebRTC stream on a video element
- adds `kind` property/attribute for track element
- adds `srcLang` property/attribute for track element

Related: #1128 #1474 #2090
http://www.w3.org/TR/html-markup/video.html says it doesn't exist, so it shouldn't in React either.