Add support for using custom rules.

test/rules/letters-E-X.js

@@ -0,0 +1,28 @@
+// @ts-check
+
+"use strict";
+
+module.exports = {
+  "names": [ "letters-E-X", "letter-E-letter-X", "contains-ex" ],
+  "description": "Rule that reports an error for lines with the letters 'EX'",
+  "tags": [ "test" ],
+  "function": function rule(params, onError) {
+    params.tokens.filter(function filterToken(token) {
+      return token.type === "inline";
+    }).forEach(function forToken(inline) {
+      inline.children.filter(function filterChild(child) {
+        return child.type === "text";
+      }).forEach(function forChild(text) {
+        var index = text.content.toLowerCase().indexOf("ex");
+        if (index !== -1) {
+          onError({
+            "lineNumber": text.lineNumber,
+            "detail": null,
+            "context": text.content.substr(index - 1, 4),
+            "range": null
+          });
+        }
+      });
+    });
+  }
+};