[Feature] Support for custom elements in SitemapStream

[tryggve]

Hi, I find this library really useful and helpful. Thank you for this project!

Describe the solution you'd like
I want the ability to extend the sitemap protocol with my own custom elements.

Additional context
According to the sitemap specification it is possible to extend the sitemap with custom elements.

There are some suggestions in the readme files that this is supported already so maybe I am missing something?

The ability to extend the xmlns with a custom namespace opens the door. Also in the API docs for SitemapStream there is a hint of this functionality where there is a custom namespace declared (commented out, but still).

But when checking the implementation for SitemapItemStream it is very bound to the known properties of the current sitemap protocol and supported extensions of news, links, videos etc.
If I try to pass in my custom properties to the stream they are simply ignored.

Thanks again for this great library.