title: --extract-audio equivalent for YoutubeDL class?
body: <p>I want to simply extract the audio in MP3 format from youtube links, but I can't figure out how to do this without calling <code>youtube-dl</code> from the command line with the <code>--extract-audio</code> option. Is there a way to do this within the <code>YoutubeDL</code> class similar to examples given <a href="https://github.com/rg3/youtube-dl/blob/master/README.md#embedding-youtube-dl" rel="nofollow">here</a>?</p>
  
  <p>Here's what I have so far, but it still creates an mp4 file.</p>
  
  <pre><code>import youtube_dl
  
  # download using optimal audio settings
  ydl_opts = {
      'format': 'bestaudio/best',
      'postprocessors': [{
          'key': 'FFmpegExtractAudio',
          'preferredcodec': 'mp3',
          'preferredquality': '192',
      }],
      'quiet': True,
      'restrictfilenames': True}
  
  ydl = youtube_dl.YoutubeDL(ydl_opts)
  ydl.download(['https://www.youtube.com/watch?v=Pnt2cy5MWyw'])
  </code></pre>
  
  <p><strong>Clarification:</strong> The above code will generate an MP3 file, but it also generates an MP4 video file. I <em>only</em> want it to generate an MP3.</p>
  
  <p><strong>Update</strong>: Here is a sample of the output when quiet is set to False:</p>
  
  <pre><code>[youtube] et6sSlEn8LE: Downloading webpage
  [youtube] et6sSlEn8LE: Extracting video information
  [youtube] et6sSlEn8LE: Downloading DASH manifest
  [download] Destination: downloaded_tracks/et6sSlEn8LE.m4a
  
  [download]   0.0% of 7.63MiB at 47.07KiB/s ETA 02:46
  [download]   0.0% of 7.63MiB at 138.74KiB/s ETA 00:56
  [download]   0.1% of 7.63MiB at 319.81KiB/s ETA 00:24
  [download]   0.2% of 7.63MiB at 679.31KiB/s ETA 00:11
  [download]   0.4% of 7.63MiB at  1.12MiB/s ETA 00:06
  [download]   0.8% of 7.63MiB at  1.19MiB/s ETA 00:06
  [download]   1.6% of 7.63MiB at  1.65MiB/s ETA 00:04
  [download]   3.3% of 7.63MiB at  2.43MiB/s ETA 00:03
  [download]   6.5% of 7.63MiB at  3.38MiB/s ETA 00:02
  [download]  13.1% of 7.63MiB at  4.32MiB/s ETA 00:01
  [download]  26.2% of 7.63MiB at  4.96MiB/s ETA 00:01
  [download]  52.4% of 7.63MiB at  5.15MiB/s ETA 00:00
  [download] 100.0% of 7.63MiB at  6.87MiB/s ETA 00:00
  [download] 100% of 7.63MiB in 00:01
  [ffmpeg] Correcting container in "downloaded_tracks/et6sSlEn8LE.m4a"
  [avconv] Destination: downloaded_tracks/et6sSlEn8LE.mp3
  Deleting original file downloaded_tracks/et6sSlEn8LE.m4a (pass -k to keep)
  </code></pre>