Add free format attribute to urlset and extend items freely.

[hinashiki]

Some people need extension, like #6 , or many media type format.
e.g.

• https://support.google.com/news/publisher/answer/74288
• https://developers.google.com/webmasters/mobile-sites/mobile-seo/configurations/separate-urls#annotation-in-sitemaps
• https://support.google.com/webmasters/answer/2620865

this pull request extend urlset attribute (we can add some attribute, ex. xmlsn:xhtml ) and add new elements to url items using callable.

sample code:

$sitemap = new Sitemap("http://hogehoge.com");
$sitemap->addUrlsetAttribute("xmlns:mobile", "http://www.google.com/schemas/sitemap-mobile/1.0");
$sitemap->addItem("/", 1.0, "hourly", date("Y-m-d H:i:s"), function(\XMLWriter $writer){
  $writer->writeElement("mobile:mobile");
});