Marker clustering support

For maps having a significant of points that are close to each other, it can be handy to let the user group the points into clusters. This issue is about making this possible as an option in the map configuration, possibly together with other parameters related to the clustering configuration itself such as: cluster icon(s), automated unclustering zoom level, grouping options etc. (depending on the configuration options that the clustering plugin offers). There are several clustering plugins implemented for Leaflet, including this one which sounds quite mature: https://github.com/Leaflet/Leaflet.markercluster