Way for only check state of files

[gnuismo]

I propose that if target folder is /dev/null then it not download files only show info...

line ~ 348 of file: /usr/local/lib/python2.7/dist-packages/gdown/download_folder.py
Add this ...

if root_folder != '/dev/null': # My contribution for not to download
    for file_id, file_path in directory_structure:
            if file_id is None:  # folder
                if not osp.exists(file_path):
                   os.makedirs(file_path)
                continue
            filename = download(
                files_url + file_id,
                output=file_path,
                quiet=quiet,
                proxy=proxy,
                speed=speed,
                use_cookies=use_cookies,
            )

            if filename is None:
                if not quiet:
                    print("Download ended unsuccessfully", file=sys.stderr)
                return
            filenames.append(filename)