Allow Stylus module to be passed as option

Sometimes a specific Stylus version is desired that is different than the one bundled with this plugin. This allows Stylus to be passed as an option to the plugin.

#### Example

If I always want to use the latest pre-1.0 version of Stylus in my project...

In `package.json`:

```json
{
  "name": "my-project",
  "dependencies": {
    "broccoli-stylus-single": "0.1.2",
    "stylus": "0.x",
    …
  }
}
```

In `Brocfile.js`:

```js
var compileStylus = require('broccoli-stylus-single');
var stylus = require('stylus');

…

var outputTree = compileStylus(sourceTrees, 'myapp/app.styl', 'assets/app.css', {
  module: stylus
});
```