Implement a diff feature to compare 2 versions of sysdiagnose

[ddurvaux]

Develop a diff feature to compare 2 snapshots of sysdiagnose and show the differences.

[dario-br]

For the moment we have a standalone command in src/sysdiagnose/utils/file_stats_diff.py

Diff command could be potentially added to the main command-line or as a separate one and add little by little diffs on parsed or analysed data.

[cvandeplas]

Very interesting feature in the context of pre/post travel.