sitemap.xml is not resolved from theme.custom_dir #335
Description
Issue
The a custom sitemap.xml template inside the theme custom_dir is not resolved properly, this is due to the get(key, default) in ExtendedPlugin.reconfigure_mkdocs_config#L157. Accesing the property custom_dir directly gives the correct path or None, currently it always resolves to None and thus searching sitemap in the working dir