title: Mapbox Marker Cluster with custom cluster icon
body: <p>I'm using Mapbox for a Phonegap app and making use of the Marker Cluster plugin to cluster our pins. Customizing the icon for a marker is easy enough, but I'd also like to be able to customize the cluster icon. I'm not seeing a way to customize the cluster icon. Suggestions?</p>
  
  <p>Here's a snippet of what I have so far:</p>
  
  <pre><code>var map = L.mapbox.map('mapView', 'myMap.mapCode', { zoomControl: false }).setView([geoData.lat, geoData.lon], 5),
      markers = new L.MarkerClusterGroup();
  
  var propertyCount = properties.length;
  for (var i = 0; i &lt; propertyCount; i++) {
      var property = properties[i];
  
      var marker = L.marker(new L.LatLng(property.lat, property.lon), {
          icon: markerIcon,
          title: property.heading
      });
  
      marker.bindPopup(property.heading);
      markers.addLayer(marker);
  }
  
  map.addLayer(markers);
  </code></pre>