Change timeout by a function

[treussart]

Replace the Timeout field with a function, which will allow me to randomize the timeout to prevent all customers from retrying in the same time frame.

[jerome-laforge]

I am not the maintainer but in order to not break the API according semver, I think that you must define TimeoutFunc func() time.Duration and let Timeout as-it. And if TimeoutFunc is not nil then use it instead of timeout else fallback on classic timeout behavior.