Radio button collections

[bradgessler]

The primitives for radio button inputs are already built into superform, but they don't quite work how Rails expects, which is to have a hidden field at the end with the current value that's set when no radio buttons are selected.

This is similar to how the select helper works, so I think they should share some code since there's a common pattern in Rails for a hidden field to be appended at the very end of a collection to serve as the "unselected" value.

Additionally, I think this code could actually be used for the select input since the current implementation doesn't assume users can select many different options.

[bradgessler]

Related to the PR at #33, which has changes requested to work with both select and input radio