use() built-in function

Stylus is extensible by the use of plugins. Currently the only way to use plugins is via the `use()` method in the JavaScript API. 

This pull request adds a simple `use()` built-in function that allows developers to use Stylus plugins declaratively:

```
use('plugins/add.js')

body
  margin add(10, 100)
```

It's basically just a pass-through to the JS API but is super convenient.