This library provides a text mode progressbar. This is typically used
to display the progress of a long running operation, providing a visual
clue that processing is underway.

The ProgressBar class manages the progress, and the format of the line
is given by a number of widgets. A widget is an object that may display
diferently depending on the state of the progress. There are three types
of widget: - a string, which always shows itself; - a ProgressBarWidget,
which may return a diferent value every time it's update method is called;
and - a ProgressBarWidgetHFill, which is like ProgressBarWidget, except
it expands to fill the remaining width of the line.

The progressbar module is very easy to use, yet very powerful. And
automatically supports features like auto-resizing when available.
