Ignore extra help flags when given with the 'help' subcommand

[natecook1000]

This resolves #259 — a bit more involved than I expected, as HelpCommand uses a custom decoder.

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