Add mapCenterOffset option.

I ended up patching the library to do something like this for Pinterest's implementation (since we have a semi-transparent sidebar that overlaps the map and want the map to center in the *actionable* area, not necessarily the center of the entire map) and wanted to see what your thoughts were on bringing it upstream.

This adds the `mapCenterOffset` option which (false by default) can be an object that contains a `paddingTopLeft` L.point and a `paddingBottomRight` L.point (if only one is supplied, it the other will be `0`). If that option is passed in to the MarkerClusterGroup, when you zoom in on a cluster, it'll offset the zoom to that delta (similar to how Leaflet's [fitbounds](http://leafletjs.com/reference.html#map-fitboundsoptions) options are).

Any thoughts?