[youtube] Add ability to authenticate with cookies

youtube_dl/extractor/youtube.py

@@ -87,7 +87,7 @@ def _login(self):
         (username, password) = self._get_login_info()
         # No authentication to be performed
         if username is None:
-            if self._LOGIN_REQUIRED:
+            if self._LOGIN_REQUIRED and self._downloader.params.get('cookiefile') is None:
                 raise ExtractorError('No login info available, needed for using %s.' % self.IE_NAME, expected=True)
             return True