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"
}