Move special directories and read special directories from config

Special directories are necessary to build the staysail source structure. So the may not behave like regular directories or page-directories. These are now defined in a method in class Util. But these are hardcoded values. Better would be for necessary directories to be hardcoded in Config class as this simplifies to understand the code (as one expects to find program steering data there). Also there are special directories, the user does not want to get error messages for, like dot-directories, usually used in Linux to hold meta files.

E.G.: ".git"

These should be loaded from the user-editable config.yaml.