Add config argument parsing capability

[agyoungs]

Use the ConfigArgParse library to support passing in rocker configurations with a config (toml, ini, yaml, etc) file.

Currently the ConfigArgParse library needs some updates, so I've put this work on hold, but as those updates are made and I make progress on this branch I figured I'd create a placeholder here.

ConfigArgParse limitations preventing forward progress on this PR:

• Inability to handle -- separation at the end of a list argument
• Out of date with latest python version
• A few bugs with multi line variables

TODO:

• unit tests
• handling several config file formats

[tfoote]

Thanks for the update and draft here for future visibilty. This looks like a great way to make it happen, but those limitations you flagged are pretty much blockers.