Feature request: Automatic renew

Hi, I've got another idea working with the lib the other day.

What do you think about adding ability to do automatic renew?
The way I see it working would be a method called something like automaticRefresh that would accept config (like normal refresh) plus existing token response (to know expiration) plus a callback that fires on refresh.

I think it's pretty straight forward, let me know what you think ;)