Add maxval argument when using progressbar as a function.

Hi! First of all, thanks for putting up an up-to-date version of progressbar on pypi since the original one is a bit borked right now.

Secondly, I often use progressbar with iterators for which I already know the length beforehand, but don't work with len(), like queries from databases, so it would be a lot handier for me if I could set the length as a positional argument when using a ProgressBar as an iterator. I know I can just set it when instantiating the ProgressBar, but I always forget the key-arg's name.