Added 'use' option to Stylus middelware Options.

Adds a ```{ use : fn }``` to 
```javascript
Stylus.midddleware({options})
```
to expose Stylus  ```.use(fn)``` capability.

E.g.:
```javascript
var autoprefixer = require('autoprefixer-stylus')();
...
app.use(stylus.middleware({
  src: __dirname + '/public'
 ,compress: false
 ,use : autoprefixer
}));
```