mirror-yt: Update output name format

We add the video height to the output file name if it's given.

We also add the playlist_index as a prefix, if the user requests this.
This is most likely a good idea, because videos uploaded on the same day
can otherwise be in another order than intended. This is most likely not
backwards compatible so already-downloaded channels should not use this.

mirror-yt.py

@@ -17,8 +17,10 @@ import click
               help="Minimum sleep interval between downloads")
 @click.option('--max-sleep-interval', type=int, default=300, show_default=True,
               help="Maximum sleep interval between downloads (see also --sleep-interval")
+@click.option('--with-playlist-index', is_flag=True, help="Prepend the playlist index to the output file name")
 @click.argument('CHANNEL_FILE', required=True)
-def download(allow_unknown, base_dir, max_video_height, sleep_interval, max_sleep_interval, channel_file):
+def download(allow_unknown, base_dir, max_video_height, sleep_interval, max_sleep_interval, with_playlist_index,
+             channel_file):
     """Mirror channels from YouTube
 
     The list of channels needs to be provided via CHANNEL_FILE. The format of the file should be one channel per line.
@@ -56,9 +58,17 @@ def download(allow_unknown, base_dir, max_video_height, sleep_interval, max_slee
         if not target_dir.exists():
             target_dir.mkdir()
 
+        # NOTE: Using %(formats.:.height)s doesn't work, because the
+        # thumbnail doesn't take it's own output template even when
+        # provided with -o 'thumbnail:…'
+        name_fmt = '%(upload_date)s - %(title)s - %(id)s{}.%(ext)s'.format(
+            f" - {max_video_height}" if max_video_height else '')
+        if with_playlist_index:
+            name_fmt = f"%(playlist_index)s - {name_fmt}"
+
         cmd = [
             'yt-dlp',
-            '-o', '%(upload_date)s - %(title)s - %(id)s.%(ext)s',
+            '-o', name_fmt,
             '--sleep-interval', str(sleep_interval),
             '--max-sleep-interval', str(max_sleep_interval),
             '--download-archive', target_dir / '.archive',