Add support for specifying stylus plugins

Here's a general purpose solution for Issue #30 that is working for me. 

For the flexibility of doing whatever you want with stylus, all you really need to do is allow the user to specify one or more stylus plugins since plugins can do anything with stylus config.

I'm using this to specify a custom url function:

```coffeescript
  connectAssets = require('connect-assets')
  connectAssets.cssCompilers.styl.use (stylus) ->
    stylus.define 'url', (node) ->
      path = node.toString().replace /['"]/g, ''
      "url('#{versionedPaths[path] or path}')"
  app.use connectAssets(...)
```

I took a look at the test suite to see if it was worth adding test for this but the suite is failing for me (mostly string == not quite matching up) and there didn't seem to be any existing tests for the stylus integration. So no test from me.