2.3 FormHelper::radio enhanced options

* add 'afterOption' and 'beforeOption' options, in order to generate more content witihn a radio set.
For instance, encapsulate the pair 'label' + 'input' within a 'li'.
* this 2 can uses the options data to generate content, through sprintf format (%1$s will be the key, while %2$s will be the title of the option), for instance adding an 'img' attached to each option.

See unit tests for examples of usage.
I'll update the documentation once these are accepted.