code: function(selected, geocoder){
      if (!geocoder._typeahead) return;
      var results = geocoder._typeahead.data;
      var selectedID = selected.id;
      var resultIDs = results.map(function (feature) {
        return feature.id;
      });
      var selectedIdx = resultIDs.indexOf(selectedID);
      return selectedIdx;
    }
docs: Get the 0-based numeric index of the item that the user selected out of the list of options
  @private
  @param {Object} selected the geojson feature selected by the user
  @param {Object} geocoder a Mapbox-GL-Geocoder instance
  @returns {Number} the index of the selected result