Support the copyFromChannel and copyToChannel methods on a WebAudio AudioBuffer
Docs: http://webaudio.github.io/web-audio-api/#widl-AudioBuffer-copyFromChannel-void-Float32Array-destination-long-channelNumber-unsigned-long-startInChannel http://webaudio.github.io/web-audio-api/#widl-AudioBuffer-copyToChannel-void-Float32Array-source-long-channelNumber-unsigned-long-startInChannel https://developer.mozilla.org/en-US/docs/Web/API/AudioBuffer/copyFromChannel https://developer.mozilla.org/en-US/docs/Web/API/AudioBuffer/copyToChannel