[MINOR] pattern: add the "ipmask()" converting function

[MINOR] pattern: add the "ipmask()" converting function

This converter can be applied on top of an IPv4-type pattern. It
applies a netmask which is suited for IP address storage and matching.
This can be used to make all hosts within a certain mask to share the
same table entries and as such use the same server.

The mask can be passed in dotted form (eg: 255.255.255.0) or in CIDR
form (eg: 24).