Add new configuration to merge cache hits

- Specifically intended to get around analytics requests with
different URLs each time, sometimes even just slightly different
hostnames!
I still quite strongly hold the other view. My example probably doesn't illustrate real requests well enough. Picture request bodies with a more realistic length of 1000 characters. And 100's of POST requests with only 10's of characters that are different in each request body (e.g. due to a datetime in the body, or a user_id, or something else trivial). I just think that kind of option will lead down the garden path. But I'm happy to agree to disagree.
Yes, I'll update the readme.