Make it possible to verify an message with an DKIM key from a text file

Currently, I see that it's only possible to load in a DKIM key from DNS in order to verify an message in dkimverify.py. As a journalist, I'd like to verify e-mail through DKIM signatures and sometimes that involves DKIM keys that might no longer be used, but I still managed to retrieve the _domainkey through some searching on the internet.

An example is here: https://github.com/DrWhax/dkim-keys/blob/master/gmail.com/20120113._domainkey.google.com/3.txt although I can't hand over any source material, obviously :)