Format the header of a tibble

For easier customization, the formatting of a tibble is split into three components: header, body, and footer. The tbl_format_header() method is responsible for formatting the header of a tibble.

Override this method if you need to change the appearance of the entire header. If you only need to change or extend the components shown in the header, override or extend tbl_sum() for your class which is called by the default method.