Move each rule implementation into its own file (fixes #83).

lib/md002.js

@@ -0,0 +1,22 @@
+// @ts-check
+
+"use strict";
+
+var shared = require("./shared");
+
+module.exports = {
+  "names": [ "MD002", "first-header-h1" ],
+  "description": "First header should be a top level header",
+  "tags": [ "headers" ],
+  "function": function MD002(params, onError) {
+    var level = params.config.level || 1;
+    var tag = "h" + level;
+    params.tokens.every(function forToken(token) {
+      if (token.type === "heading_open") {
+        shared.addErrorDetailIf(onError, token.lineNumber, tag, token.tag);
+        return false;
+      }
+      return true;
+    });
+  }
+};