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

[dstufft]

These supports the RFC 5861 extensions to cache control which allow a site to tell a HTTP cache that it's acceptable to serve a stale response while re-validating the currently cached response (stale-if-revalidate) and that it's acceptable to serve a stale response if an error occurs (stale-if-error). These both take a number of seconds for which it is acceptable to serve a stale response for.

I didn't write any tests because there doesn't seem to be any that exercise all of the various header options, if this needs tests let me know.

[bertjwregeer]

Thanks :-).

Tests will need to be written eventually for all of the header options, just to make sure that as I start making changes I don't break anything.

[dstufft]

Do you want me to add them to this PR for these headers or would you prefer to just do them all at the same time?

[bertjwregeer]

If you have the time to add tests and think you know where it is appropriate to add them, it is less work I have to do later. I was mainly adding it as a mental note on this PR for myself.

[bertjwregeer]

Thanks!