Feature request:Yubikey 2-factor support?

I know right now maybe it's not top priority, but I wanted to put it out there, whenever you get the chance, maybe show the Yubikey a little love and add 2-factor support for online operations (plus of course the usual 2-factor software usual suspects, e.g. Google Authenticator)? I think second factor eventually should be added and it makes sense that a strong, hardware based competitor like the yubikey could be in the mix. I feel the crypto community would feel glad if you did.

The issue requests Yubikey and Google Authenticator support for online operations, but names no files, entry points, or acceptance criteria. First map the existing authentication flow and clarify which operations and second-factor methods are in scope; done would require an agreed design and tests for the selected methods.

Written by the indexing model from the issue text.