Allow to load rules from subdirectories

[denis-sokolov]

When a custom rule grows, it becomes clumsy to keep it in one file. Moving it into a subdirectory still requires an entry file, resulting in two similar, but distinct names in the custom rules directory.

This PR allows a shape of module directories that’s native to Node.js packages: a subdirectory with an index.js file in it.

[not-an-aardvark]

Thanks for the PR! Unfortunately I don't have time to maintain this repository anymore. I would recommend forking the repository if you want to use new features like this.