[Wish] Support pre-JSON CLI output

[fleimgruber]

Following https://github.com/CLIP-HPC/SlurmCommander#feedback a wish for SC to support older Slurm installations. I read the part about "Hard requirement: json-output capable slurm commands", but at the same time wondered how hard it would be to craft a small and confined intermediate layer that would just parse the output of the basic CLI commands sinfo, sacct, squeue, ... to construct the equivalent JSON from the newer API and feed that to the existing SC components?

I saw that you are already forward-looking with #23 and that this is more of a backward-looking thing, call it legacy SC, but I am sure there are a lot of ancient Slurm installations out there - we run with Slurm 18.08.