re-add the functionality to add custom stlyes

i wanted to add custom styles like it is described [here](http://docs.typuscmf.com/customization/custom_css_and_js.html) in order to use jquery.chosen on a form element. this requires custom javascript (still works) AND custom styles (currently disabled).

just for example purposes:

```sass
# app/assets/stylesheets/typus/custom.sass
@import "chosen"
```
```coffee
# app/assets/javascripts/typus/custom.coffee
//= require "chosen.jquery"
jQuery ->
  $('select#event_user_id').chosen(search_contains: true)
```