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

youtube_dl/__init__.py

@@ -9,6 +9,7 @@ import codecs
 import io
 import os
 import random
+import shlex
 import sys
 
 
@@ -255,6 +256,9 @@ def _real_main(argv=None):
             xattr  # Confuse flake8
         except ImportError:
             parser.error('setting filesize xattr requested but python-xattr is not available')
+    external_downloader_args = None
+    if opts.external_downloader_args:
+        external_downloader_args = shlex.split(opts.external_downloader_args)
     match_filter = (
         None if opts.match_filter is None
         else match_filter_func(opts.match_filter))
@@ -359,6 +363,7 @@ def _real_main(argv=None):
         'no_color': opts.no_color,
         'ffmpeg_location': opts.ffmpeg_location,
         'hls_prefer_native': opts.hls_prefer_native,
+        'external_downloader_args': external_downloader_args,
     }
 
     with YoutubeDL(ydl_opts) as ydl: