whitelist clients in dnswl.org

First, thanks for all of your work on postgrey.

I'm wondering if you'd be open to a patch like this that whitelists any sender IP in the [dnswl.org](http://dnswl.org/) DNS-based whitelist.

I'd be glad to investigate the false positive rate in my own mail (impressionistically it looks like 5% of DNSWL mail turns out to be spam). And I'd also be glad to revise the patch to make this optional and/or defaulted off.