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

lib/md041.js

@@ -10,6 +10,7 @@ module.exports = {
   "names": [ "MD041", "first-line-heading", "first-line-h1" ],
   "description": "First line in a file should be a top-level heading",
   "tags": [ "headings" ],
+  "parser": "markdownit",
   "function": function MD041(params, onError) {
     const level = Number(params.config.level || 1);
     const tag = "h" + level;