Support for using JSON record values in redis key name

We needed to use parts on the JSON record in the key name, so I’ve added support for this. Feel free to rewrite my solution or disregard it, but I feel that this is a very useful addition to increase the flexibility in terms of what can be used to construct the key name.

The main change is that my solution sends the JSON record to the get_count_key method, which is then used to format the key according to the new syntax (%_{key_name}), and after that according to any time formatting options.

I’ve updated the README, but not CHANGELOG and VERSION.