add serve-index for directory listings

package.json

@@ -56,6 +56,7 @@
     "nat-pmp": "0.0.3",
     "node-acme": "0.0.1",
     "request": "^2.53.0",
+    "serve-index": "^1.6.2",
     "serve-static": "^1.9.1",
     "ssl-root-cas": "^1.1.7",
     "urlrouter": "^0.5.4",