Add support for showing custom error messages

[vadlvs]

Linting requirements can vary from project to project. For instance in our case we have a requirement to show an error message when the number of scenarios exceeds some number. It would be nice to provide an extension point to achieve these kind of requirements.
Can you please let us know if you have anything like this already available / planned for it?

We have gone through the lib/gherkin-linter file and found that onParse callback is already available. This sounded fitting our use case as the callback function do receive cucumber messages object which can be used to lint and generate custom messages but looks like the output of onParse is not being used today. Can put in a PR if you think this method can be used for this purpose. Please share your thoughts!