title: Cluster markers from different overlays with each other
body: <p>I use angular-leaflet with leaflet markercluster plugin (see <a href="http://tombatossals.github.io/angular-leaflet-directive/examples/0000-viewer.html#/layers/overlays-markercluster-example" rel="nofollow noreferrer">http://tombatossals.github.io/angular-leaflet-directive/examples/0000-viewer.html#/layers/overlays-markercluster-example</a> for example). </p>
  
  <p>Now I have three different overlays with 'clustermarker' type. At the moment markers of each overlay cluster only with markers from the same overlay. I would like all of them to cluster with each other. How can I accomplish this?   </p>