args :: [String.t | number, ...] | String.t | number
redix_result :: {:ok, Redix.Protocol.redis_value} | {:error, atom | Redix.Error.t}
exec(args, String.t) :: redix_result
exec!(args, String.t) :: Redix.Protocol.redis_value
pipeline_exec([args]) :: redix_result
redis :: pid