Having rel="alternate" URLs in sitemap under custom circumstances

This patch is intended to help those that want to enchance xmlsitemap in similar ways as the above image. This solution is closely tied to a large custom project, and not meant to be direclty applied to the module.

While working on a drupal project that had a rather nifty multi-region implementation (todo: future blog), it was required to have a single sitemap for all regions with default and alternate urls in it, as suggested by google - https://support.google.com/webmasters/answer/189077

None of the modules that extend xmlsitemap fit the bill -

• domain_xmlsitemap module - https://www.drupal.org/project/domain_xmlsitemap
• xmlsitemap_i18n module - comes within the xmlsitemap download
• this issue trying to address the same thing - #1670086: using rel="alternate" rather than multiple sitemaps by language context

After exploring all options, the custom changes submitted in this issue were made. Hope this helps others. (NOTE: code has been simplified to work for anybody, and show what's going on)