stylus asset takes an additional option for manual configuration

I needed a way to use [bootstrap-stylus](https://github.com/shomeya/bootstrap-stylus) with the stylus asset so I made a small modification to allow manually configuring the stylus compiler via an additional option.

I opted to use ```options.config?.call styl styl``` so that it would look good in both coffeescript and javascript.

eg: coffeescript
```coffeescript
new StylusAsset
  url: '/style.css'
  filename: __dirname + '/style/fun.styl'
  config: ->
    @use bootstrap()
    @define 'setting', 90
```

eg: javascript
```javascript
new StylusAsset({
  url: '/style.css',
  filename: __dirname + '/style/fun.styl',
  config: function (stylus) {
    stylus // using this here seems a little unnatural
      .use(bootstrap())
      .define('setting', 90);
  }
});
```