simplify

not sure why this module has so much going on. for the array stuff, you could probably just do:

``` js
slug(':title-by-:author')
```

Is the function stuff necessary? I guess we could just do:

``` js
slug(function(model) { ... })
```

... and whenever there is a change event, recompute.

Permission to simplify / break the API? :-)

/cc @rschmukler 
