ui: support CIDR notation for specifying source addresses

shotgun.lua supports --bind-pattern and --bind-num to specify the source addresses, which is very simple implementation-wise, but not very usable or user-friendly.

A new option (mutually exclusive with the previous ones) should be introduced (perhaps some like --bind-network?) which would take a CIDR notation as input and would calculate a suitable distribution of source IPs from this network segment among individual senders/threads.