Add radio type support to woocommerce_form_field

Even though the core itself does not use the `radio` type much, plugins sometimes may want to use the `radio` field type and since `woocommerce_form_field` is a general form field generator function, I think it makes sense to add support for this type as well.

This PR simply adds support for the `radio` type. As it stands, it does not suggest any styling, but it does add radio-button specific classes to the options and their labels.