Adds creationText option to display a line in the results to indicate it will create a new keyword

A client asked me to add a line in the results to show that the queried keyword will be created as a new keyword if submitted. We agreed that the result expressed in this commit was okay for him.

It will "override" emptyText unless the queried keyword have already been added to the selection.

I've added specs and a demo.

What do you think ?