customcomplete.py example appears broken

[johnw42]

I see there's a variable called PREAMBLE that looks like it contains code to make the rest of the completion logic work, but it's not used anywhere. It looks like it was lost in commit 5c6a63a. It looks like the easiest fix would be to add a preamble argument to add_argument_to and arrange for its value to be passed through to complete.