Supporting Asciidoc

Hello, I have raised a question in the Github repository about supporting Asciidoc syntax similarly to how it currently works with Markdown.

This new feature will allow anyone to implement their own "flavor" so that in future any format could be supported - following the "open / close" principal.

It will also allow users to use advanced site building tools like "antora" to create living documentation.

This feature will also help rework the old "cukedoctor" repo https://github.com/rmpestano/cukedoctor/issues/306 since it does not follow the new gherkin messages format.

We had some discussion with the repo maintainers and I ended up proposing Pull Requests and an example repo to illustrate how it will work.

I would like to ask for some discussion, ideas and reviews and  on the implementation since I did not get any from the github project :(. 

PRs:

https://github.com/cucumber/gherkin/pull/95

https://github.com/cucumber/messages/pull/139

Discussion:

https://github.com/cucumber/gherkin/issues/10

Thank you for your help and participation