Add --format json option to keys, stats, and check commands - #11

Summary

Adds an opt-in --format json flag to print output in json format. The plain output format remains the default. It facilitates the output to be processed to other programs if needed.

Code changes

• Create a shared format_output function. Kept flexible for adding new format types.
• --format is an optional flag on the keys, stats, and check commands. Choices are bounded for it in the subparsers using choice=['plain', 'json']

Testing

• Modified the FakeExtraArgs, and FakeCheckArgs classes to accommodate the format flag.
• Adds 5 test, pass successfully with the pre-existing tests. Test coverage is at 98.14%, meeting project's spec.