added on-add-point-to-cluster even
added on-add-point-to-cluster even

added on-add-point-to-cluster event to manipulare cluster attributes
during cluster creation. This can be useful to customize renderers not
only on count basis

Esri/cluster-layer-js,Esri/cluster-layer-js,ssylvia/cluster-layer-js,ssylvia/cluster-layer-js