Add a variant of IPRange that does not mask the IP automatically

When dealing with IPv6 interface addresses it is commonplace (at least in Linux) to specify them in the IP/prefix format to define IP and netmask in one go. I was going to simply use the Read instance of IPRange to parse these interface addresses but since IPRange masks the IP down to a netid I can't use it to store this configuration data and had to use a custom parser instead.