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!
You shouldn't be POSTing datetimes :)  Sounds like a wonky API design on the other end if that's expected.  If we run into issues in the future with this, we should probably go down the route of removing certain configurable keys from the request body.  Definitely not worth tackling until it's a problem though.

Re: README: While that's true, the line you modified is only for POSTs.  Either way, it's still clear enough.  I'll test this out locally soon and get it in!