[AVFS] Option to show the avfs #-dirs

Hi,

First of all, thanks for the great tool!

Second, have you thought about adding an option to show the magic avfs
directories ending with `#` symbol next to each file with supported
extension?

It will make it possible to use avfs with graphical file managers and
other systems which expect all available directories to be visible with
readdir/ls call (like overlayfs).

I made such patch for myself and it seems to work fine for the case of
overlayfs mounted on top of avfs - all changes made inside readonly
archives are stored in overlayfs "upper" dir, so for other apps it looks
like all archives are mounted read-write!

However, my code is rather dirty (list of supported extensions is
hardcoded and there is no option to switch this off), that's why I'd like
to know maintainers' opinion before creating a pull request (or how is it
done on sourceforge): are you interested in adding such functionality to
the mainline avfs?

Thanks,

Aleksei.