closesTryGhost/Team#2094
This value can be used for non-crucial data with a `public` option. For example: `public, max-age=1, stale-while-revalidate=9`.
The idea behind this option is that the browser would cache the value for the number of seconds in `max-age` and would use it for the number of seconds in `stale-while-revalidate` until it gets a "validated response" from the server. The behaviour should be almost unnoticeable for the end user but would make a big difference in the amount of requests to server.
refs https://github.com/TryGhost/Toolbox/issues/363
- this middleware is standalone and I suspect we're going to be touching
it further when we work on Ghost's caching in the near future
