A small bug fix (an improvement too)

Hi Craig,

Thanks a lot for the effort of writing this neat and sweet extension!
It is simple, however useful and robust.
It shows IPv6 addresses on my Linux box! :D

However, after a while, I find it occasionally showing "null" IP
when I visit `mail.google.com`.

So I read the code and figured that it would be better to maintain a dict
of domain names and IPs, instead of URLs and IPs.
What do you say? :)

Best,
Jialong