code: function command(os) {
    var z = 'ffmpeg';
    var os = os||[];
    for(var o of os) {
      var o = o||{};
      for(var k in o) {
        if(o[k]==null) continue;
        if(k==='stdio') continue;
        if(k==='o' || k==='outfile') z += ` "${o[k]}"`;
        else if(typeof o[k]==='boolean') z += o[k]? ` -${k}`:'';
        else z += ` -${k} ${JSON.stringify(o[k])}`;
      }
    }
    return z;
  }
docs: Generate command for ffmpeg.