add list operations

[r2evans]

Thanks for the basic functionality! I have use for ordered pubsub queues, so I wanted to add list operations to the functions available. I started with https://redis.io/commands#list and skipped "blocking" functions.

Redis functions implemented: LINDEX, LINSERT, LLEN, LPOP, LPUSH, LPUSHX, LRANGE, LREM, LSET, LTRIM, RPOP, RPOPLPUSH, RPUSH, RPUSHX.

NB: I added crude tests, mostly due to time and infamiliarity with pytest and the existing structure. While they work, it's likely they could be improved or at least formalized better than I've done here. They are not automatically run, ready at the moment for manual/interactive testing.