Nagios alerts support has been added via NSCA (Nagios Service Check Acceptor).
Nagios NSCA alerts configuration can be set at runtime via the
preferences page of the UI. The running instance of ntopng is
notified asynchronously upon preference changes.
This commit adds a first implementation of configuration preferences for RRDs.
Different config options are selectable for network interfaces RRDs and
other RRDs. This addresses issue #33.