Add stale-while-revalidate option

[wi-ry]

As per https://support.cloudflare.com/hc/en-us/articles/115003206852-Understanding-Origin-Cache-Control:

Cache-Control: max-age=600, stale-while-revalidate=30 This configuration indicates the asset is fresh for 600 seconds and can be served stale for up to an additional 30 seconds to parallel requests for the same resource while the initial synchronous revalidation is attempted.