Add more options to the "resume" setting

[joezuntz]

This opens up the "resume" setting, which currently can be set to True or False.

Now you can also set it to

• "resume" - to continue the pipeline where you left off (same as True)
• "restart" - to ignore existing files and restart the pipeline at the beginning (same as False)
• "refuse" - to refuse to start the pipeline if any output files already exist.

[eacharles]

This all looks good. At some point you might consider using and enum and tweak the config stuff to treat it as a choice.