Cache variant of HTTP::Async #22

@evdb / @kaoru - I would love to help make a Cache (HTTP::Async::Cached) sub module that will cache responses for the HTTP clients to reduce the throttling this module can do.

Any ideas on how to do that? I am reading the code base and wanted to get your feedback on the best way to do it?

Love this module btw, use it often for a lot of my web tools. It is amazing fast and versatile. Kudos.