code: function buildSitemapIndex (conf) {
    var xml = [];
    var lastmod;
  
    xml.push('<?xml version="1.0" encoding="UTF-8"?>');
    if (conf.xslUrl) {
      xml.push('<?xml-stylesheet type="text/xsl" href="' + conf.xslUrl + '"?>');
    }
    if (!conf.xmlNs) {
      xml.push('<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" ' +
        'xmlns:mobile="http://www.google.com/schemas/sitemap-mobile/1.0" ' +
        'xmlns:image="http://www.google.com/schemas/sitemap-image/1.1" ' +
        'xmlns:video="http://www.google.com/schemas/sitemap-video/1.1">');
    } else {
      xml.push('<sitemapindex ' + conf.xmlNs + '>')
    }
  
    if (conf.lastmodISO) {
      lastmod = conf.lastmodISO;
    } else if (conf.lastmodrealtime) {
      lastmod = new Date().toISOString();
    } else if (conf.lastmod) {
      lastmod = new Date(conf.lastmod).toISOString();
    }
  
  
    conf.urls.forEach(url => {
      if (url instanceof Object) {
        lastmod = url.lastmod ? url.lastmod : lastmod;
  
        url = url.url;
      }
      xml.push('<sitemap>');
      xml.push('<loc>' + url + '</loc>');
      if (lastmod) {
        xml.push('<lastmod>' + lastmod + '</lastmod>');
      }
      xml.push('</sitemap>');
    });
  
    xml.push('</sitemapindex>');
  
    return xml.join('\n');
  }
docs: Builds a sitemap index from urls
  
  @param   {Object}    conf
  @param   {Array}     conf.urls
  @param   {String}    conf.xslUrl
  @param   {String}    conf.xmlNs
  @return  {String}    XML String of SitemapIndex