Add support for stringification to setex-style headings
Add support for stringification to setex-style headings

yukkurisinai/mdast,ulrikaugustsson/mdast,tanzania82/remarks,ulrikaugustsson/mdast,chcokr/mdast,eush77/remark,eush77/remark,wooorm/remark,yukkurisinai/mdast,chcokr/mdast