allow to create new tokens on the fly

in respones to:
https://github.com/manuelbernhardt/jquery-tokeninput/commit/fb7af54fb2e46b42e5b1ef1aa40087b2bfc1eb51#commitcomment-360625

If you like this enhancement, I'd suggest to add to other options: one for the hint (right now it's a hardcoded 'new'), the other would be an onCreate callback, which would create a possibility to send a post request to the server when creating a new token.

I'd prepare both and send another pull request, if you consider to add this functionality.

Cheers!