Add Rule.parser property for custom rules to specify which Markdown parser output they use, honor it strictly, and add full type support.

lib/md025.js

@@ -11,6 +11,7 @@ module.exports = {
   "names": [ "MD025", "single-title", "single-h1" ],
   "description": "Multiple top-level headings in the same document",
   "tags": [ "headings" ],
+  "parser": "markdownit",
   "function": function MD025(params, onError) {
     const level = Number(params.config.level || 1);
     const tag = "h" + level;