set opt(val) {
    this._opt = Object.assign({}, {
      'element': '',
      'className': '',
      'exclude': [],
      'iframes': false,
      'iframesTimeout': 5000,
      'separateWordSearch': true,
      'acrossElements': false,
      'ignoreGroups': 0,
      'each': () => {},
      'noMatch': () => {},
      'filter': () => true,
      'done': () => {},
      'debug': false,
      'log': window.console
    }, val);
  }
Options defined by the user. They will be initialized from one of the
public methods. See {@link Mark#mark}, {@link Mark#markRegExp},
{@link Mark#markRanges} and {@link Mark#unmark} for option properties.
@type {object}
@param {object} [val] - An object that will be merged with defaults
@access protected