Add js and example for 'none of the above'

[edwardhorsford]

It's somewhat common to have a 'none of the above' checkbox with a set of radios. I've seen several services do this. With js we can have it uncheck existing options, or uncheck itself if other options are picked.

It's not much js, but might be a nice touch to add to the kit. A possible downside is users might not find it - should we have an example?

We built this in to our temporary event notice prototype - code here.

[NickColley]

This looks like a interesting feature but not something I've seen anywhere else, if there is a need here please consider raising an issue on the backlog and we get think about getting this in not just in the kit.

alphagov/govuk-design-system-backlog#37

Thanks again :)