Insert arrays of chunks in multistream

This would be very useful, when you want to append/prepend some cached data.

---

Like this:

```js
var streams = [
  fs.createReadStream(__dirname + '/numbers/1.txt'),
  ['Bird', 'bird'],
  fs.createReadStream(__dirname + '/numbers/2.txt'),
  function () { return ['Bird', 'is', 'the', 'word'] },
  fs.createReadStream(__dirname + '/numbers/3.txt')
]

MultiStream(streams).pipe(process.stdout) // => 1Birdbird2birdistheword3
```