Allow some configuration through composer.json extra config

[geerlingguy]

I would like to allow a few settings to be configured through a Drupal project's composer.json extras config, e.g.:

    "extra": {
        "drupal-vm-docker": {
            "container_name": "custom-container-name",
            "http_port": "80",
            "https_port": "443",
        }
    }

Basic things like that which will be templated into the docker-compose.yml file. Or maybe do something else here, but basically I want a few configurable flags.

[frob]

I would be interested to see some DX review on this. I have always preferred per-project configuration to be in a separate file --such as config.yml. I have this preference because to start a new project I like to copy the composer.json and change the least amount of stuff possible.