Add ability for grouped layers to be exclusive.
Add ability for grouped layers to be exclusive.

Simple modification that allows to specify exclusive layer groups by adding the
name of the group to to options.exclusiveGroups.
This resolves ismyrnow/Leaflet.groupedlayercontrol#2
and can be used to gain the functionality discussed in Leaflet/Leaflet#593

ismyrnow/Leaflet.groupedlayercontrol,msiadak/Leaflet.groupedlayercontrol,msiadak/Leaflet.groupedlayercontrol,ismyrnow/leaflet-groupedlayercontrol,ismyrnow/Leaflet.groupedlayercontrol,ismyrnow/leaflet-groupedlayercontrol