Improve user experience, make job executor mandatory positional argument

[wilke]

I see you added required=True, but what I meant (although I didn't explain it well, sorry) was to make it a mandatory positional argument, so that usage would look like psij-consol run slurm spec.json rather than psij-consol run spec.json -j slurm.

Originally posted by @jameshcorbett in #162 (comment)