[websockets] Add WebSocketFragmentFD (#399)

Necessary for #392

Co-authored by: nao20010128nao, pukkandan

yt_dlp/downloader/__init__.py

@@ -24,6 +24,7 @@ from .rtsp import RtspFD
 from .ism import IsmFD
 from .mhtml import MhtmlFD
 from .niconico import NiconicoDmcFD
+from .websocket import WebSocketFragmentFD
 from .youtube_live_chat import YoutubeLiveChatReplayFD
 from .external import (
     get_external_downloader,
@@ -42,6 +43,7 @@ PROTOCOL_MAP = {
     'ism': IsmFD,
     'mhtml': MhtmlFD,
     'niconico_dmc': NiconicoDmcFD,
+    'websocket_frag': WebSocketFragmentFD,
     'youtube_live_chat_replay': YoutubeLiveChatReplayFD,
 }
 
@@ -52,6 +54,7 @@ def shorten_protocol_name(proto, simplify=False):
         'rtmp_ffmpeg': 'rtmp_f',
         'http_dash_segments': 'dash',
         'niconico_dmc': 'dmc',
+        'websocket_frag': 'WSfrag',
     }
     if simplify:
         short_protocol_names.update({