Mixin support

Hi, I'd like to be able to use [mixins](http://tjholowaychuk.com/post/7590787973/jade-mixins-includes) in my Jade code that are imported in from another directory. Is this already possible or is it not supported? For example in my main Jade code:

``` jade
include mixins/track
.track
    mixin trackView(data)
```

Then in my mixins/track.jade:

``` jade
mixin trackView(data)
    p= data.title
```
