Some objects you might want to show progress on while iterating over
do not support calling len(). In these cases you can now pass
`expected_size` in to the progress.bar and progress.mill functions
to avoid the len() call on the iterable.
I've created a progress indicator that outputs a "mill" and added it to
the `progress.py` file.
Very simple stuff but its more compact than the other progress bars and
it can be useful when using long labels.
Don Spaulding