Pass kwargs config to redis.Redis()

References #12;

Pass kwargs to `redis.Redis()` in the same pattern as flask-cache.

Tests passing (for the default config), but no new tests added in this PR. I don't see a very easy way to test these changes without mocking redis, which I guess is out of the scope of this PR.
That would be a lot simpler :+1: Not like we are setting any defaults that aren't already present in `redis.Redis` anyways.