Fix f-hidden? to work with subdirectories

[paradoja]

Currently f-hidden?'s implementation doesn't work with subdirectories, even when the documentation ( https://github.com/rejeep/f.el#f-hidden-path ) shows it should be otherwise.

This PR fixes it and changes the tests so that they test on subdirectories.

[paradoja]

Not sure though, if any subdirectory of a hidden directory should be considered hidden too.

[Phundrak]

I added some commits to your branch in order to update the tests with the current state of the package.

However, I’m also unsure whether directories and files inside a hidden directory should be considered as hidden or not, I’d like to hear about other people before merging it.

[Phundrak]

I tried reimplementing f-hidden-p on branch https://github.com/rejeep/f.el/tree/feature/better-f-hidden-p in a way that would not completely break the current API, I’d like to hear some opinions about it