Add CanStore option

For more context, see https://developer.chrome.com/docs/web-platform/bfcache-ccns

Support stale cache entries

To support eTag/change detection for entries marked stale outside of this library.

Also switch the values in XETag1 and XETag2 so they are ordered by old/new.

Add ShouldCache option

Downgrade Go version in go.mod

Add AlwaysUseCachedResponse option func

Misc adjustments

* Rework (some) tests
* Add EnableETagPair option to add a pair of eTags, even if the server does not provide one
* Add CacheKey option
* Remove redundant nil check
* Optionally allow caching for other HTTP methods than GET and HEAD

Add AroundRoundTrip option, trim the API

Remove unsed, add fork info, run gomod init etc.