allow style preprocessors to pass options

If you add a css preprocessor (in your app's Brocfile.js) with the following code, the options are never passed to the underlying plugin. This PR fixes that.

```js
app.registry.add('css', 'broccoli-stylus-single', 'styl', {
  paths: [
    __dirname + '/vendor',
    __dirname + '/vendor/bootstrap-stylus/stylus'
  ]
});
```

---

This PR addresses my concern on https://github.com/stefanpenner/ember-cli/issues/810#issuecomment-47247365 but does not resolve it completely.

Test failure appears to be a problem already on master. No additional tests fail as a result of this change.
Can you remove the `only`?