Request: Exit with code other than 0 if command-line args are malformed

[Alex Dewar]

We use markdownlint-cli all over the place in our CI system via the pre-commit hook (thanks for making it, btw!) and I've just noticed that the hook isn't actually running in a bunch of our repositories, because we're passing in dud command-line arguments, namely we've forgotten the terminating -- after the list of rules to disable. markdownlint still terminates with exit code 0 though, so the pre-commit hook still passes. It would be better if it returned a non-zero exit code so that users don't accidentally break the hook like we did.

[David Anson]

Duplicate of #104 (and see related issues)