[downloader/external] Add support for custom options (Fixes #4885, closes #5098)

youtube_dl/downloader/common.py

@@ -42,6 +42,8 @@ class FileDownloader(object):
     max_filesize:       Skip files larger than this size
     xattr_set_filesize: Set ytdl.filesize user xattribute with expected size.
                         (experimenatal)
+    external_downloader_args:  A list of additional command-line arguments for the
+                        external downloader.
 
     Subclasses of this one must re-define the real_download method.
     """