Feature - Add sitemap articles

[Loocos]

Hi there,

I would suggest to add provider for the sitemap xml for the articles and categories using the @stefandoorn plugin (https://github.com/stefandoorn/sitemap-plugin).
If u didn't have time, I'll try to create a PR in my free time.

Hello,
Feel free to contribute if you have time. This plugin will be free forever.
Thanks!