add build:watch

package.json

@@ -6,6 +6,7 @@
   "scripts": {
     "test": "echo \"Error: no test specified\" && exit 1",
     "build": "webpack",
+    "build:watch": "webpack --watch",
     "start": "webpack-dev-server -d --hot --config webpack.config.js --watch"
   },
   "keywords": [],