Adding support for recursive file filtering without including empty directories.

[Mason Mackaman]

I came up with an approach to fix the empty directory problem. I wanted to make you aware of it in case you'd be able to use it to improve the UX.
NixOS/nixpkgs#188301.

[Jonas Chevalier]

thanks. It looks like a similar idea as #9 (comment)
where the algorithm first traverses the filesystem using builtins.readDir, and then applies the filtered result to builtins.path. It's a good idea.