title: Python download youtube with specific filename
body: <p>I'm trying to download youtube videos with <code>pytube</code> this way:</p>
  
  <pre><code>from pytube import YouTube
  YouTube('http://youtube.com/watch?v=9bZkp7q19f0').streams.first().download()
  </code></pre>
  
  <p>but the file will have the same name as the original video name. How do I specify a custom filename?</p>