Add suffix to ProgressReport as an attribute and document it
The progress library supports very nice syntax for the suffix of progress bars. We should add a suffix attribute to a ProgressReport and encourage plugin writers to provide a suffix string. This way each progress report can be rendered exactly how they want their progress to be shown. This field should default to None.
The ProgressReport class docstring should be updated with examples. I think copying the table from the progress documentation would be good, but maybe we should just link to the documentation.