Optionally, do not force the first choice when the enter key is hit

This is something that came in handy for me.

**Steps to reproduce:**

1. Go to the [example site](http://andymatthews.net/code/autocomplete/)
2. Type "m"
3. Hit the enter key

**Result:** "Maine" is forcefully chosen as the search term.

Now, I can imagine cases where this would be desired (e.g. a list of the 50 states) and cases where this would not be desired (e.g. a general-purpose search engine where the user types in ```basket``` and the first suggestion is ```basketball```).

So I'm suggesting adding an option to disable it.