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.