Option to scan specific CIDRs

This commit adds the option -z which receives a CIDR. 
Blacklist is not yet working for this option.

Example:

sudo zmap -p 80 -N 10 -B 1M -o - -z 192.168.1.1/24
Aug 19 17:55:21.098 [INFO] zmap: started
Aug 19 17:55:21.103 [INFO] send: Processing CIDR 192.168.1.1/24
Aug 19 17:55:21.103 [INFO] send:  First IP 192.168.1.1
Aug 19 17:55:21.103 [INFO] send:  Last IP 192.168.2.0
192.168.1.1
192.168.1.15