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

test/rules/first-line.js

@@ -7,6 +7,7 @@ module.exports = {
   "names": [ "first-line" ],
   "description": "Rule that reports an error for the first line",
   "tags": [ "test" ],
+  "parser": "none",
   "function": function rule(params, onError) {
     // Unconditionally report an error for line 1
     onError({