Stream support

I want to mock a superagent request that creates pipes the data to a write stream e.g.

```js
const targz = require('tar.gz2');
const request = require('superagent');

const tarball = 'some-file.tgz';
const read = request.get(tarball);
const write = targz().createWriteStream(dir);

const stream = read.pipe(write).on('finish', doSomething);
```