title: MapBox GL custom markers with symbol layer
body: <p>I am trying to cluster custom markers in MapBox GL JS but I cannot figure out how to get a custom marker image from a url into the symbol layer? It either does not work or no markers show up at all.  How is it done? I need to know how to use a custom image from a url with the symbol layer. Thank you very much.  </p>
  
  <pre><code>map.addSource('parcelpoints', {
          type: 'geojson',        
          data: geojson,
          cluster: true,
          clusterMaxZoom: 14, // Max zoom to cluster points on
          clusterRadius: 50 // Radius of each cluster when clustering points (defaults to 50)
      });
  
      map.addLayer({
          id: 'clusters',
          type: 'circle',
          source: 'parcelpoints',
          filter: ['has', 'point_count'],
          paint: {
              // Use step expressions (https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-step)
              // with three steps to implement three types of circles:
              //   * Blue, 20px circles when point count is less than 100
              //   * Yellow, 30px circles when point count is between 100 and 750
              //   * Pink, 40px circles when point count is greater than or equal to 750
              'circle-color': [
                  'step',
                  ['get', 'point_count'],
                  '#51bbd6',
                  100,
                  '#f1f075',
                  750,
                  '#f28cb1'
              ],
              'circle-radius': [
                  'step',
                  ['get', 'point_count'],
                  20,
                  100,
                  30,
                  750,
                  40
              ]
          }
      });
  
      map.addLayer({
          id: 'cluster-count',
          type: 'symbol',
          source: 'parcelpoints',
          filter: ['has', 'point_count'],
          layout: {
              'text-field': '{point_count_abbreviated}',
              'text-font': ['DIN Offc Pro Medium', 'Arial Unicode MS Bold'],
              'text-size': 12
          }
      });   
  
          map.addLayer({
          id: 'unclustered-point',
          type: 'symbol',
          source: 'parcelpoints',
          filter: ['!has', 'point_count'],
          'layout': {
           'visibility': 'visible',
           'icon-image':  { url: "marker.svg" }
          }
      });
  </code></pre>