option for private files analogous to dotfiles

I don't know if this would even work because I don't know if Windows has something like this, but I'm wondering if you would be open to adding an option (or accepting a PR) to handle private files (files that do not have world-readable) in the same way that `dotfiles` is handled.

A public file or directory is typically mode `644` or `755`

A private file or directory is typically mode `400` or `500`

My suggestion is that an option like `privatefiles: 'ignore'` would treat any file lacking world-readable permission (the `o` of `ugo` does not have 4) the same as `dotfiles: 'ignore'` is currently treated.