code: function(selected){
      // clean up any old marker that might be present
      this._removeMarker();
      var defaultMarkerOptions = {
        color: '#4668F2'
      }
      var markerOptions = extend({}, defaultMarkerOptions, this.options.marker)
      this.mapMarker = new this._mapboxgl.Marker(markerOptions);
      this.mapMarker
        .setLngLat(selected.center)
        .addTo(this._map);
      return this;
    }
docs: Handle the placement of a result marking the selected result
  @private
  @param {Object} selected the selected geojson feature
  @returns {MapboxGeocoder} this