Re: [whatwg/fetch] Body.arrayBuffer([begin, end]) (#554)

You can create an `AudioBuffer` using `OfflineAudioContext` containing audio data between specific time ranges. This will allow a sample of audio data to be played even where the specified range of playback does not begin at `0`.

```
async function audioSample({url, from, to, when = 0, channels = 2, sampleRate = 44100}) {
  const duration = to - from;
  const [ac, oac] = [new AudioContext(), new OfflineAudioContext(channels, sampleRate * duration, sampleRate)];
  const request = await fetch(url);
  const response = await request.arrayBuffer();
  // this should only be necessary once to get `AudioBuffer`
  const data = await ac.decodeAudioData(response);
  const source = oac.createBufferSource();
  source.buffer = data;
  source.connect(oac.destination);
  // create an `AudioBuffer` of audio between `from` and `to`
  source.start(when, from, duration);
  const ab = await oac.startRendering();
  return {ab, ac, oac}
}

// create an `AudioBuffer` of media content from 60 to 65 seconds
audioSample({url:"https://ia600305.us.archive.org/30/items/return_201605/return.mp3", from:60, to:65}) 
.then(({ab, ac, oac}) => {
  console.log(ab, ac, oac);
  const source = ac.createBufferSource();
  source.buffer = ab;
  source.connect(ac.destination);
  // close `AudioContext` 
  // https://developer.mozilla.org/en-US/docs/Web/API/AudioContext/close
  source.onended = event => ac.close().then(() => console.log(event, ac));
  source.start();
});
```

[plnkr](http://plnkr.co/edit/QLZ2ju0DavdGNg2iOYof?p=preview) 

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