{% formconfig field ... for "<widget class name here>" %}

Hi,

I added support for using widget classes as a filter for the `{% formconfig field ... %}`.

The usecase for this is to specify extra attributes for textareas. Textareas are usually used on `CharField`s, so we have no way yet of passing extra arguments to those templates. With the proposed formconfig filtering this is possible:

```html+django
{% form form using %}
    {% formconfig field with extra_classes="span12" for "Textarea" %}
    ...
{% endform %}
```