New feat: show change statistics/anaytics when comparing text

[houfu]

In #11, there was some discussion on showing some stats on the changes detected. Stats are always welcomed especially if they are calculated by computers.

Per @HRNPH:

LGTM, maybe some stats on Diff Count Would be good longest diff shortest diff you'll never know when it'll come in handy Learned this from my NLP job lol

This stat should also be considered.

% change

These features should probably be implemented:

• [ ] A function to calculate the statistics (return a dictionary?)
• [ ] Add an output stats function, and/or extend the existing output_markdown and output_rich functions to include the stats
• [ ] Add tests
• [ ] Add docs

Please feel free to discuss or suggest further.