package.json: add commands to build extension for prod and development

Author: Kocal

template/package.json

@@ -4,7 +4,8 @@
   "description": "{{ description }}",
   "author": "{{ author }}",
   "scripts": {
-    "build": "webpack --config webpack.config.js --progress --hide-modules",
+    "build": "cross-env NODE_ENV=production webpack --config webpack.config.js --progress --hide-modules",
+    "build:env": "cross-env NODE_ENV=development webpack --config webpack.config.js --progress --hide-modules",
     "build-zip": "node scripts/build-zip.js",
     "watch": "npm run build -- --watch"
   },
@@ -19,6 +20,7 @@
     "babel-loader": "^7.1.1",
     "babel-preset-latest": "^6.24.1",
     "copy-webpack-plugin": "^4.0.1",
+    "cross-env": "^5.0.5",
     "css-loader": "^0.28.4",
     "extract-text-webpack-plugin": "^3.0.0",
     "file-loader": "^0.11.2",