generate-domains-blocklist.py does support single IP but not CIDR

[ManuHry]

Hi 😄 generate-domains-blocklist.py is very useful to merge lists
Unfortunately, it cannot handle CIDR (generates an empty section), could be a great feature!

Example use case: import Spamhaus Don't Route Or Peer Lists (DROP) and Indicators of Compromise lists with IPv4 and IPv6 CIDR in blocked-ips.txt file that supports CIDR also
This allows to kind-of block malicious networks for devices that are not protected by any firewall (partial mitigation)