queue .use()

I tried to fix https://github.com/segmentio/nightmare/issues/10. Not really sure if this is the right way but it's the simplest I have come up with. Basically we just run each plugin seperate and call the next function in the main queue when the given plugin is done.

```js
new Nightmare()
  .use(first())

function first () {
  return function (nightmare, done) {
    nightmare
      .goto() // etc ...
      .run(done);
  }
}
```

Would love to hear what you think and if it makes sense to `.run()` each plugin. @reinpk