Fix crash on initial run when no configuration exists

[nilbus]

jutebag.commands used to be an object with command names as the keys, until e69f7dfc90ffcbcf82f313de863be47590861f64 changed jutebag.commands from an object to a sorted array.

Use an appropriate lookup, now that jutebag.commands is an array.

Also, make bin/jutebag executable, so it can be run directly, without explicitly running node while developing.

[shedali]

this works for me, thanks :+1:

[MaxPleaner]

Thanks this pull request fixed it for me.

[nilbus]

@akoenig What do you think about merging and releasing this fix, André?