title: kill all child_process when node process is killed
body: <p>How do i make sure all child_process are killed when the parent process is killed. 
  I have something like the below one. </p>
  
  <p>Even when the node process is kill i see that FFMPEG continues to run and the out.avi is generated. How can i stop FFMPEG from running after the node process exits. </p>
  
  <pre><code>var args = "ffmpeg -i in.avi out.avi"
  child_process.exec(args , function(err, stdout,stderr){});
  
  child_process.exec(args , function(err, stdout,stderr){});
  </code></pre>