Stop parsing earlier when a catch-all argument is present

[natecook1000]

If a command defines an @Argument property with the .unconditionalRemaining parsing strategy, we need to stop parsing input when we encounter either a positional argument or an unrecognized option/flag label. Note that this is a change in behavior, as seen in the modified test.

Resolves #331.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[natecook1000]

@swift-ci Please test