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 ;)

No files or tests are named. Start by reviewing the React Native bridge and its AppAuth refresh flow, then determine how a proposed automaticRefresh method could use refresh configuration, an existing token response, and a refresh callback. Done should include an agreed design and behavior for automatic renewal.

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