add option for s-maxage inside cacheControl

Hi Guys,

I use deployment on Zeit now. I would like to set the `s-maxage` to enable CDN response but it seems this option is not available. It would be great if `cacheControl` would be more flexible to set this option.

Especially options like `stale-while-revalidate` would be great to add.

Thanks

No file, test, or entry point is named in the issue. Locate the cacheControl handling and its existing tests first, then determine how s-maxage and stale-while-revalidate should be configured; done means these response-cache directives can be set without breaking current cacheControl behavior.

Written by the indexing model from the issue text.