New ignorecommand with path to external command for dynamic ignoreip list

Introduce new ignorecommand with path to external programm output of which is added to the ignoreip list.
Allows you to get a dynamic ip list that do not need to ban. For example, this can be a list of ip from a nearby radius server or a dynamic list of email relays.
It is expected that the command returns a list of ip separated by a space.
Author of patch: Aleksander Aleksandrov <hazg@mail.ru>