Add support for authoring custom rules.

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
+    });
+  }
+};