Add build script

Author: danekszy

package.json

@@ -5,6 +5,7 @@
   "main": "index.js",
   "scripts": {
     "start": "parcel index.html",
+    "build": "parcel build src/index.js",
     "dev": "node dev-server.js",
     "test": "npm run test:lint && npm run test:unit",
     "test:watch": "jest --watch src/",