* Add support for custom encode and decode keys (91). There are now two new callbacks that can be registered: `decode_key_loader` and `encode_key_loader`. The decode callback is passed in the **unverified** JWT claims, and must return a string that will be used to decode and verify the JWT. The encode callback is passed in the identity (as passed in to the `create_access_token` or `create_refresh_token` functions) and must return a string that will be used to encode a JWT. If unset, the `JWT_SECRET_KEY`, `JWT_PUBLIC_KEY`, or `JWT_PRIVATE_KEY` will still be used as appropriate.