Support stale-while-revalidate and stale-if-error

Hi @kornelski, thanks for your work on this library, it looks fantastic.

For my use case, I would need the library to support these two Cache-Control directives. From MDN:

• stale-while-revalidate=<seconds>: Indicates that the client is willing to accept a stale response while asynchronously checking in the background for a fresh one. The seconds value indicates for how long the client is willing to accept a stale response.
• stale-if-error=<seconds>: Indicates that the client is willing to accept a stale response if the check for a fresh one fails.

I think it could be implemented in the following way:

• satisfiesWithoutRevalidation() would check the stale-if-error directive and return true if the response is a 5XX and the stale response can be used.
• A new method satisfiesWithRevalidation() would check the stale-while-revalidate directive and return true if the stale response can be used while revalidating in the background. The caller would be expected to make the request while also using the cached response.

If you’re open to these enhancements, I’d be happy to work on them.

[kornelski]

If you’re implementing it in parallel with an actual HTTP client I’m confident it’ll make sense.

[sithmel]

The implementation is ready here: https://github.com/kornelski/http-cache-semantics/pull/30