Provide prefix-preserving anonymization option

[sit]

IP prefix preservation can be useful for analyzing the structure of network traffic. A pure HMAC-MD5 approach loses this structure. It may be useful to have such an option.

See recommended best practices from the Center for Applied Internet Data Analysis and the CryptoPAn algorithm.

[vbrown608]

Cool, thanks @sit! I'm making my way through the original CryptoPAn paper. Looks like someone wrote an implementation is Go as well.

Based on the linked best practices, I think it would also be nice to provide an option to not anonymize special IP address ranges, ex. not globally reachable.