Merge branch 'bug/168' into 'master'

Add OptionManager#parse_known_args

*Description of changes*

Add `parse_known_args` to our `OptionManager` interface so plugin flags can be specified. This provides similar behaviour to argparse's `parse_known_args` method on its `ArgumentParser`. When we transition to argparse, we'll be able to take direct advantage of that.

*Related to:*  #168

See merge request !74