title: FFmpeg loglevel with level prefix
body: <p>I want to add level prefix to ffmpeg logs.
  Im using <code>ffmpeg -loglevel level+info -i input</code>
  And it is working as well in my Mac (ios) but is not working in my Ubuntu 18 with ffmpeg version 3.4.6-0ubuntu0.18.04.1</p>
  
  <p>When I am running it in my Ubuntu I got:</p>
  
  <pre><code>Invalid loglevel "level+info". Possible levels are numbers or:
  "quite"
  "panic"
  ...
  ...
  ...
  "trace"
  </code></pre>
  
  <p>Please advise</p>