add cidr2ips method

[mingoooo]

I have a requirement, cidr convert to a list of ip address.This is my code, maybe someone needs it.

[hamishforbes]

Cool, that seems useful!

The only thing I would say is if the cidr doesn't parse we should probably return the error rather than logging it unconditionally.
The only place we log rather than return is when parsing multiple cidrs, this is so the other potentially valid cidrs are still returned.

A couple basic tests would be good too!