Feature suggestion: allow setting a subset of configuration

[codeclown]

For my project I had to set a custom value for httpPort in pebble-config.json.

Currently in my custom JSON I need to specify the rest of the configuration as well:

{
  "pebble": {
    "listenAddress": "0.0.0.0:14000",
    "managementListenAddress": "0.0.0.0:15000",
    "certificate": "test/certs/localhost/cert.pem",
    "privateKey": "test/certs/localhost/key.pem",
    "httpPort": 80,
    "tlsPort": 5001,
    "ocspResponderURL": "",
    "externalAccountBindingRequired": false
  }
}

It would be convenient if I could omit all properties but the ones I need to modify. So my JSON would look like this:

{
  "pebble": {
    "httpPort": 80
  }
}

Pebble could pull the default values for the rest.

[cpu]

Seems like a reasonable request 👍 I probably won't have a chance to implement this short term but would accept a PR from the community.