code: function selectHeadings (contentSelector, headingSelector) {
      var selectors = headingSelector
      if (options.ignoreSelector) {
        selectors = headingSelector.split(',')
          .map(function mapSelectors (selector) {
            return selector.trim() + ':not(' + options.ignoreSelector + ')'
          })
      }
      try {
        return document.querySelector(contentSelector)
          .querySelectorAll(selectors)
      } catch (e) {
        console.warn('Element not found: ' + contentSelector); // eslint-disable-line
        return null
      }
    }
docs: Select headings in content area, exclude any selector in options.ignoreSelector
  @param {String} contentSelector
  @param {Array} headingSelector
  @return {Array}