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 think that kind of config might be more than what we need for what we're trying to achieve here. Maybe I've misunderstood what you're suggesting, but it sounds like your trying to cater for this case with that extra config:

Consider the following 3 POST requests:
```
:: Request 1 ::
POST http://www.example-analytics.com/user/SDF879932/

x = 1
y = 1

:: Request 2 ::
POST http://www.example-analytics.com/user/OIWEMLW39/

x = 1
y = 1

:: Request 3 ::
POST http://www.example-analytics.com/user/KJAHDI898/

x = 99
y = 99
```

My current solution gives you the ability to ensure that although the 3 requests have *different URLs*, I can see that it's just application data in the URL that is changing. So for the purpose of automated tests, I'm happy to use the same persisted cached response.

It sounds like you're proposing to provide the ability to distinguish between the first 2 requests, and the last one, because some more application data is different, namely, the content of the body of the request.
It may be true that someone may need that use case. But considering that the URL themselves are different (which is of more significance than the request payload), and yet a regex is defined to match them all, I think it's an unlikely use case.