Generate JSON output

[devdanzin]

This proof of concept adds a --json option to diff, index, rank and report, allowing to generate output in JSON format.

Example output (excerpt from wily report -n 11 --json src\wily\commands\report.py):

[
 {
    "Revision": "f1e8225",
    "Author": "Anthony Shaw",
    "Date": "2023-03-12",
    "Cyclomatic Complexity": "24 (0)",
    "Unique Operands": "8 (0)",
    "Maintainability Index": "56.8982 (0.0)",
    "Lines of Code": "206 (0)"
  },
  {
    "Revision": "e9921dd",
    "Author": "Christian Clauss",
    "Date": "2023-03-11",
    "Cyclomatic Complexity": "24 (0)",
    "Unique Operands": "8 (0)",
    "Maintainability Index": "56.8982 (0)",
    "Lines of Code": "206 (0)"
  }
]

Pasting this here I just realized it might be a good idea to add file name to each JSON entry. Oh well, back to the drawing board.

Still needs tests and docstring updates.

Fixes part of #92.