Other RBLs?

Issue Summary

I'm wondering if it would be a good idea, and would this module be a good framework, to add support for other blacklists such as:

http://bsb.empty.us/
http://dsbl.org/main
http://www.spamhaus.org/xbl/

The past few days I've seen some enormous spam-runs on my site, with hundreds of different IPs, very few of which http:BL caught but an open proxy dbl would probably have.