Tell user when there are more Typeahead results than `items` option allows

Created by: reefdog

The items option on Typeahead limits the menu to items-number of menu items. Which is great, but optionally we should alert the user with a "+ n more" bit of text at the bottom of the menu when there are more results than items allows to be shown.

I propose an option, show_item_overflow_note or something less verbose, which if true puts a live "+ n more results" text snippet below the last item.