title: merge multiple video files and stream by fluent-ffmpeg in nodejs
body: <p>I am trying to merge multiple video files into stream output using fluent-ffmpeg. But it only streams the first video.</p>
  <pre><code>res.set('content-type', 'video/webm');    
  var proc = ffmpeg()
          .mergeAdd('file-1.webm') // tried with input()
          .mergeAdd('file-2.webm')
          .mergeAdd('file-3.webm')
          .format('webm')
          .size('680x?')
          .on('end', function() {
            console.log('file has been converted succesfully');
          })
          .on('error', function(err) {
            console.log('an error happened: ' + err.message);
          })
          // save to stream
          .pipe(res, {end:true})
  </code></pre>
  <p>Any ideas?</p>