Add automatic backend detection

[camall3n]

Cam: "I think by default it should check for slurm / gridengine, and run one of those, with an override like --local if you need that"
Neev: "Nah, it should keep the --backend override and just check for a default at runtime. It's more flexible this way"
Cam: "Oh so you're saying it would automatically build a config using the available backend?"
Neev: "Well it could. But we can also just set the default= in argparse to a function evaluated at runtime or something"

[camall3n]

The auto-backend feature should print out the backend it thinks you wanted to use