code: function (source, options) {
          this.index = this._geIndex(source);
          this.data = '';
          this.options = options;
          this._cache = {};
          this._usedHeaders = [];
      }
docs: Constructor
  @param {String} source
  @param {Object} [options]
  @param {Number} [options.maxDepth]
  @param {Char}   [options.bullet]