* youtube-dl: add download the best audio+video
With this command the user can download the best audio and video separately. After it's done downloading, it will spawn ffmpeg to merge the two files together. Handy when downloading stuff from youtube as they host video and audio separate.
* youtube-dl: removed braces
Not so sure about the last example. I wanted to give an example of how youtube-dl allows you to very precisely filter a playlist, so I made up a sort of strange task. It might be better to just remove it though.
