Facilitate overwriting of Consumer class

[AndreCimander]

This is just a very minor nitpick, feel free to close it, but it would be nice to change the class of the Consumer without the need to completly replicate and overwrite .consume() :)

[Bogdanp]

This seems OK to me, but how about making it a class property instead?

[AndreCimander]

I was thinking about adding it as a cls property, but the consumer classes are not yet defined.

We could go the @property route, though, this would also make it easier to add logic if one desires. How would you like it named? consumer / consumer_class? :)

    @property
    def consumer_class(self):
        return _RedisConsumer

[Bogdanp]

Ah, right, that makes sense. Yeah, consumer_class sounds good.

[Bogdanp]

Thanks! I have squashed and merged this.