Enable xdebug and allow configuration from yaml

We should enable the xdebug extension. This should be as easy as including the existing [xdebug class from our puppet module](https://github.com/zivtech/puppet-drupal-php/blob/48058b7ad34b7a2ec4dedb7566d57da47b1180a5/manifests/extension/xdebug.pp).
