Run external command for download/streaming

[etmatrix]

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I think should be useful permit to run external command for download url. For example I can run vlc or mplayer for see a video. Sure we can use pipe but with pipe we can't seek, I have to download all the video, for big video or slow network this is a problem.
I open this issue, but I will try to submit a patch for this new feature

[dstftw]

--external-downloader COMMAND    Use the specified external downloader.
                                 Currently supports
                                 aria2c,avconv,axel,curl,ffmpeg,httpie,wget
--external-downloader-args ARGS  Give these arguments to the external
                                 downloader