FFMpeg support in youtube-dl

This article was originally published at https://gist.github.com/joepie91/ce9267788fdcb37f5941be5a04fcdd0f. It may no longer be necessary.

Based on this post:

{
  # ...
  stablePackages = with pkgs; [
    # ...
    (python35Packages.youtube-dl.override {
      ffmpeg = ffmpeg-full;
    })
    # ...
  ];
  # ...
}

(To understand what stablePackages is here, see this entry.)