Add a folder source to scan recursively from path (string).

        :param path: folder path.
        :param depth: if provided will be depth limit. 0 = first level only.
        :param source_type: what to return; files only, folders only, or both.

def add_folder(self, path, depth=None, source_type=DefaultSourceType):
        """
        Add a folder source to scan recursively from path (string).

        :param path: folder path.
        :param depth: if provided will be depth limit. 0 = first level only.
        :param source_type: what to return; files only, folders only, or both.
        """
        self.add_source(FolderSource(path, depth, **source_type))
        return self