Add dev command that watches file changes

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

package.json

@@ -30,8 +30,9 @@
     "webpack-merge": "^4.1.2"
   },
   "scripts": {
-    "test": "./node_modules/karma/bin/karma start karma.conf.js --single-run",
-    "build": "./node_modules/webpack/bin/webpack.js --config js/webpack.prod.config.js"
+    "build": "./node_modules/webpack/bin/webpack.js --config js/webpack.prod.config.js",
+    "dev": "./node_modules/webpack/bin/webpack.js --config js/webpack.dev.config.js --watch",
+    "test": "./node_modules/karma/bin/karma start karma.conf.js --single-run"
   },
   "repository": {
     "type": "git",