code: function ffmpeg(os) {
    var stdio = os.stdio===undefined? STDIO:os.stdio;
    return new Promise((fres, frej) => cp.exec(command(os), {stdio}, (err, stdout, stderr) => {
      if(err) frej(err);
      else fres({stdout, stderr});
    }));
  }
docs: Invoke "ffmpeg" asynchronously.
  @param {object} os ffmpeg options.