Allow AutoComplete to select first item from result on a specific key press

Right now the autocomplete requires you to press Down arrow to select an item.

Instead if you partially type a word, and you get results, after pressing Tab key (or another key of your choice, e.g. Enter or Space) the first item in the list is selected and chosen as the selection.

Very similar to the Tag box on StackOverflow: http://stackoverflow.com/questions/ask