Modify headers on each retry

[Christopotamus]

Is there any way to modify the headers before each retry?

I have an auth token which expires and must be refreshed, but having tenacioius-fetch retry on 401 seems to be sending the same old token. It would be super helpful if we can replace headers on each retry.

[tiaanduplessis]

@Christopotamus I agree that it would be helpful. Wil look into implementing ASAP 😄