The only one I could find: https://github.com/mivok/markdownlint, but not sure how mature it is.
- the more cross platform the better
- the more free the better
I want this to enforce my style guide.
mdast-lint https://github.com/wooorm/mdast-lint has come out, and it looks like a serious possibility.
You can configure it both per repository, and globally, and it covers many of the style guide points I wanted in my style guide.