https://github.com/rg3/youtube-dl
http://apple.stackexchange.com/questions/26099/is-there-a-way-to-convert-audio-files-in-mac-os-x-or-the-command-line-without-us
youtube-dlインストール
mp4ダウンロード
mp4->mp3変換
brew install youtube-dl youtube-dl https://www.youtube.com/watch?v=Y4Mop8L13wA ffmpeg -i input.mp4 output.mp3
