feat(OverlayView): allow rendered as children of MarkerCluster

* Let `OverlayView` be clusterd in `MarkerClusterer`
* Thanks to @ziad-saab 
* Closes #187

src/creators/OverlayViewCreator.js

@@ -169,6 +169,21 @@ export default class OverlayViewCreator extends Component {
       }
     };
 
+    // If we're inside a MarkerClusterer, allow ourselves to be clustered
+    if (overlayViewProps.anchorHolderRef) {
+      if (`MarkerClusterer` === overlayViewProps.anchorHolderRef.getAnchorType()) {
+        overlayView.getDraggable = function getDraggable() {
+          return !!overlayViewProps.draggable;
+        };
+
+        overlayView.getPosition = function getPosition() {
+          return new google.maps.LatLng(this.position);
+        };
+
+        overlayViewProps.anchorHolderRef.getAnchor().addMarker(overlayView);
+      }
+    }
+
     return overlayView;
   }