Bind nato-region in embark-encode-map

[hmelman]

nato-region was added to emacs 24 and converts the region to the NATO Alphabet.

I bind it to n in embark-encode-map (and N to denato-region) and I think it could be useful to others.

I don't use it often, but I do find it useful at times when talking on the phone and needing to spell something. So that's way more often than I use morse-region, or studlify-region which are already bound in embark-encode-map.