Add an option to select only testing docstrings/standalone files #71

I have a repository where I have some literate documentation/testing Markdown files next to my code, but I don't have any tests in docstrings. Therefore, I would like to have pytest only collect .md files, and avoid collecting all .py files and checking the docstrings for tests.

My request would be to add options to allow this, e.g. add --markdown-docs-only-docstings and --markdown-docs-only-text which disable the other type of collection.

If appreciated, I can add this and open a PR :)