combineVideos() {
    const { ccommand } = this;

    return new Promise((resolve, reject) => {
      const dir = this.rootConf('detailedCacheDir');
      forEach(this.list, video => {
        const { ss, to } = this.conf;
        ccommand.input(video);
        if (ss && to) {
          ccommand.addInputOption('-ss', ss);
          ccommand.addInputOption('-to', to);
        }
      });
      ccommand.on('start', commandLine => console.log(commandLine));
      ccommand.on('end', () => resolve());
      ccommand.on('error', (error, stdout, stderr) => reject([error, stdout, stderr]));
      ccommand.mergeToFile(this.getPath(), dir);
    });
  }
Combine multiple videos into one
@private