JSON output support

[rsgalloway]

add support for machine-friendly JSON output:

$ envstack secrets -o baked.env --json

could output a list of dicts like:

{
  "key": "SUBFORK_ACCESS_KEY",
  "exists": true,
  "empty": false,
  "value": "abc123.....xyz456",
  "source": "/etc/project/env/secrets.env"
}

[rsgalloway]

revisiting this, the example command:

envstack secrets -o baked.env --json

makes no sense to me: the file ext is .env but it's content is json? this might make sense:

envstack secrets -o baked.json

but I think the use case for this is debugging and/or for machine consumption. perhaps a better flag for this would be --dump, where envstack dumps out everything it knows about how the environment was built.