Supervisor for Partisan's broadcast groups.

Starts the groups declared by configuration (see partisan_broadcast:child_specs/0) and supports adding or retiring groups at runtime via supervisor:start_child/2 and terminate_child/2 (driven by partisan_broadcast:start_group/1 / stop_group/1). Each child is an independent partisan_plumtree_broadcast instance, so one group crashing or restarting does not disturb the others.