Add optional progress bar indicators

[bkryza]

For large diagrams with many translation units it can be frustrating to wait several minutes without any update for the diagram. It would be nice to have an option --progress, which would show progress bar indicators, one progress bar for each diagram, the progress of each bar would be proportional to the number of translation units already traversed for that diagram.

A good candidate could be: https://github.com/p-ranav/indicators