与其传递mp4文件的位置，不如传递ytdl流作为源，就像这样:

Instead of passing the location of mp4 file, pass in the ytdl stream as the source, like so:

stream = ytdl(url)

proc = new ffmpeg({source:stream})
proc.setFfmpegPath('/Applications/ffmpeg')
proc.saveToFile(mp3, (stdout, stderr)->
            return console.log stderr if err?
            return console.log 'done'
        )