Add support for RFC5861 Cache-Control headers

[calpaterson]

RFC5861 standardised two Cache-Control extensions:

1. stale-while-revalidate
2. stale-if-error

Please add support for these in the standard werkzerg ResponseCacheControl.

One use case of these extensions is to allow sites to have their CDNs "coalesce" requests for content when it goes stale. Another is to serve a stale representation from the CDN when the site is having an outage. It's possible to solve this problem without changes to werkzeug, but as it's a standard feature, I think it would be nice to have it in the library.