Add an easy API in the templates to add a class to an input element.

This issue was brought up recently by #86 and is quite a basic use case. So we need to come up with:
- [ ] an API how to add classes to form widgets.

Start by reviewing issue #86 and the existing template API for form widgets. Define how a template can add classes to an input element, then document the expected usage and verify that the resulting widget markup exposes the requested classes.

Written by the indexing model from the issue text.