Add an argument to url_check to yield non-zero exit status for bad URLs

[cicdguy]

As a follow-up from #39, it would be imperative to add an argument to url_check that would yield a non-zero exit code if one or more bad URLs are found.

Use case: In a CI/CD workflow, we would want to simply run urlchecker::url_check() and let the workflow fail to alert the user that one or more URLs are non-conformant.