Add `hostname` option to `SitemapIndexStream` [Feature]

**Describe the solution you'd like**
It would be great if `SitemapIndexStream` would also accept the `hostname` option, just as `SitemapStream` does.

I think this would make for a more consistent API.

If this is something the maintainers would think is a good addition, I would be more than happy to submit a PR.

Let me know! :)