Validation of CIDR-notated IPv4 and IPv6 ranges #117

First of all, thanks for writing this library! It's been extremely helpful and very easy to use.

It'd be nice to have support for validating CIDR-notated IPv4 and IPv6 ranges.

My understanding of the CIDR format is that it's just a valid IPv4 or IPv6, followed by a /, followed by a number between 0 and 32 (for IPv4) or 0 and 128 (for IPv6). The RFC backs this interpretation up: https://tools.ietf.org/html/rfc4632

This is something I could work on at some point, although I have limited cycles at the moment. This issue just serves to indicate interest, in case someone else feels like tackling it 😄