add Rememerable#after_remembered callback

I started looking into adding `ActiveSupport::Notification.instrument`s to some devise actions in an app. I found plataformatec/devise#2592. When digging a little deeper, I found some callbacks that I'd never seen before that _could_ be useful to inject instruments into my app.

They aren't comprehensive, and the documentation is disappointing.

This PR is a first stab at improving that.

I'm used to rspec, so excuse my test_unit tests, I've never written tests this way before. I've tried to match the style.

I intend to keep looking into instrumentation.