Add support for authoring custom rules.

test/rules/any-blockquote.js

@@ -3,8 +3,8 @@
 "use strict";
 
 module.exports = {
-  "names": [ "blockquote" ],
-  "description": "Rule that reports an error for blockquotes",
+  "names": [ "any-blockquote" ],
+  "description": "Rule that reports an error for any blockquote",
   "tags": [ "test" ],
   "function": function rule(params, onError) {
     params.tokens.filter(function filterToken(token) {

test/rules/first-line.js

@@ -0,0 +1,18 @@
+// @ts-check
+
+"use strict";
+
+module.exports = {
+  "names": [ "first-line" ],
+  "description": "Rule that reports an error for the first line",
+  "tags": [ "test" ],
+  "function": function rule(params, onError) {
+    // Unconditionally report an error for line 1
+    onError({
+      "lineNumber": 1,
+      "detail": null,
+      "context": null,
+      "range": null
+    });
+  }
+};

test/rules/rules.js

@@ -2,17 +2,21 @@
 
 "use strict";
 
-var blockquote = require("./blockquote");
-module.exports.blockquote = blockquote;
+var anyBlockquote = require("./any-blockquote");
+module.exports.anyBlockquote = anyBlockquote;
 
 var everyNLines = require("./every-n-lines");
 module.exports.everyNLines = everyNLines;
 
+var firstLine = require("./first-line");
+module.exports.firstLine = firstLine;
+
 var lettersEX = require("./letters-E-X");
 module.exports.lettersEX = lettersEX;
 
 module.exports.all = [
-  blockquote,
+  anyBlockquote,
   everyNLines,
+  firstLine,
   lettersEX
 ];