Allow fuzzier IP matching

[rcsheets]

In order to better preserve user privacy, I would like to use this plugin but have it store only partial IP addresses rather than the entire IP.

One way to do this would be to store and match on just the first 24 bits of an IPv4 address, or first 60 bits of an IPv6 address. The prefix length could be configurable.

Another possibility would be to look up the origin AS or netblock and match on that.

Finally, a match on a geoip lookup could also work. This seems more complex than either of the above, but would allow for notifications that might make sense to more users. For example, a notification of unusual activity would make more sense to most users if it included the city from which the unusual activity originated.

Do any of these approaches sound interesting? I'd be happy to collaborate on this and contribute code provided we can agree on a good approach, and if this is a feature you'd consider merging.