Added ability to customise the 'parameters' key.

This allows you to use something that isn't `parameters` as the top-level key in your resultant output. Add the following parameter in your `composer.json`:

    "extra": {
        "incenteev-parameters": {
            "parameter-key": "config"
        }
    }
 
replacing `config` with your required key.  Defaults to `parameters` if nothing is supplied.