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