Added functionality to render sitemap.xml

Signed-off-by: Ilja

Hello.
I found very useful to generate sitemap file for the generated static website. Sitemap is used by search engines, like Google. To get more information, please refer to [the official resource](https://support.google.com/webmasters/answer/156184?hl=en&ref_topic=8476).

I also updated _base.zip_ archive and added default template **sitemap.ftl**.