OfflineAudioContext

[`OfflineAudioContext`](https://developer.mozilla.org/en-US/docs/Web/API/OfflineAudioContext) is a great feature to process audio data without its rendering layer.

```js
var offlineContext = require('audio-context/offline');
```

Let me know if you think it would be better to implement it in a different fashion.

PS: it relies on #2 to be merged.

![](http://i.imgur.com/rnhlFXy.gif)