Add markdownlint formatter

The currently available formatter for Markdown (Flexmark) is a bit intrusive for my taste, especially on tables. On the one hand, it might help to actually implement more of the configuration options. But from my feeling, [markdownlint ](https://github.com/DavidAnson/markdownlint) would be a nicer alternative. I already use it as a vscode-plugin, but integrating it into spotless would be even better. As there already is some infrastructure to call npm packages, it should be reasonable to implement.