Rebase on `vs.xml`

When https://github.com/lazy-eggplant/vs.xml is going to be production-ready, it might be good to rebase `vs.templ` on it.  
It offers a better handling of namespaces and native support for some transformation which are harder to get right with `pugixml`.  
Not a priority as the current version works just fine. It will be probably v2.x