Allow resumption

[kpcyrd]

It should be possible to stop the audit at a position and then resume at that position. I'm not yet sure how that should be implemented:

• Credentials can get requeued, which means they get pushed to the end of the queue, making it harder to save a single numeric position
• We might need a "save file" dialog, which would require some work to be nice to use (tab completion for paths?)
• We could add a "stop" command, that, if --continuable has been set, drains the remaining attempts into a save file that we can pick up again
• The approach above needs to identify the scripts as well. we currently don't have a unified way to identify a script

This issue supersedes #3