Add support for specifying the target generator config file

src/fixtures/nuxt/async.mjs

@@ -0,0 +1,15 @@
+const getAllDynamicRoute = async function() {
+  const routes = await (async () => {
+    return ['/posts/hello-world', '/posts/hello-again'];
+  })();
+  return routes;
+};
+
+export default {
+  mode: 'universal',
+  generate: {
+    async routes () {
+      return getAllDynamicRoute();
+    }
+  }
+};