Submodule Path Broken

I noticed that your submodule points to a GitHub repository that no longer exists. As this prevented the project from building, I replaced it with [vmg/sundown](https://github.com/vmg/sundown.git).

I know that vmg/sundown is deprecated; however, it is the first Sundown repository that came to mind. If you know of a better one, don’t hesitate to use it instead of the one I’m suggesting.