code: function generateHeadingSelector(headingIndexingLevel) {
    let headingsSelectors = ['.always-index:header', 'h1'];
    for (let i = 2; i <= headingIndexingLevel; i += 1) {
      headingsSelectors.push(`h${i}`);
    }
    headingsSelectors = headingsSelectors.map(selector => `${selector}:not(.no-index)`);
    return headingsSelectors.join(',');
  }
docs: Generates a selector for headings with level inside the headingIndexLevel
  or with the index attribute, that do not also have the noindex attribute
  @param headingIndexingLevel to generate