Can't configure file store path with $sitemap->store()

Created by: Charkhan

Hey,

Just ran into this problem : I want to generate sitemaps with big data that I will put via SFTP on another repo.

To do so, I need to put the sitemaps on a specific folder to let another task upload files to the public website.

I wish to be able to do so :

$sitemap->store($format, $filename, $pathToStore)

I can do a PR if you like the idea, I think there is not much to change.