downloading YT videos & converting to mp3.md

1. download & install youtube-dl (https://github.com/rg3/youtube-dl/blob/master/README.md#readme) (for mac: brew install libav & brew install youtube-dl)
2. add the following function to your shell env:

ytdl() {
  youtube-dl -x --audio-format=mp3 --no-playlist -o "$1.%(ext)s" $2
}

3. use it like so: ytdl "Hello - World" http://link.to/youtube-vid (playlist can be appended, it will be ignored)