function loadByID(id) {
    var iconConfig = config[id];
    return loadByURL(iconConfig.url).then(function(icon) {
      return new Icon(icon, iconConfig);
    });
  }
Lookup the configuration in the registry, if !registered throw an error
otherwise load the icon [on-demand] using the registered URL.
@param {string} id icon registry id
@returns {angular.$q.Promise}