aceholder, which is the current heading level
            * submenu_class (string) :   ''   - add custom class(es) for each child group of headings; has support for '%level%' placeholder which is the current "submenu" heading level
            * base_url      (string) :   ''   - add a base url to the TOC links for when your TOC is on another page than the actual content
            * anchor_class  (string) :   ''   - add custom class(es) for each anchor element
            * skip_no_ids   (bool)   : false  