Making the path extension generated in xml, be optional

By default the `<loc>` is value the `file.html`.
Eg:

````xml
<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9 http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd"
	xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
<url>

  <!-- the change influence this value -->
  <loc>foo.com/bar.html</loc>
  <!-- ------------------- -->

  <lastmod>2014-12-30T02:38:50.000Z</lastmod>
  <changefreq>daily</changefreq>
  <priority>0.5</priority>
</url>
</urlset>
````

I'm inserting a **extension attribute** that by default is true but if it is changed to false, the `.html` shall be replaced by `/`
Eg:
````xml
<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9 http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd"
	xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
<url>

  <!-- value changed when extension is false -->
  <loc>foo.com/bar/</loc>
  <!-- ------------------- -->

  <lastmod>2014-12-30T02:38:50.000Z</lastmod>
  <changefreq>daily</changefreq>
  <priority>0.5</priority>
</url>
</urlset>
````