Feature: Support human-readable stamps in output

[joshuatz]

For quickly scanning stamps, it is hard to tell at a glance what something like this actually means in terms of relative dates:

{
  "md/docs/contributing.md": {
    "created": 1605291448,
    "modified": 1606105609
  }
}

It would be nice to support an option that adds to (not replaces) the existing fields with human readable dates. For example,

{
  "md/docs/contributing.md": {
    "created": 1605291448,
    "modified": 1606105609,
    "created-readable": "Friday, November 13, 2020 10:17:28 AM GMT-08:00",
    "modified-readable": "Sunday, November 22, 2020 8:26:49 PM GMT-08:00"
  }
}

Thought would have to be given to how to deal with timezones.