chore(package): update html-webpack-plugin to version 4.2.1

Closes #13

Author: greenkeeper[bot]

package.json

@@ -28,7 +28,7 @@
     "eslint-plugin-jsx-a11y": "^6.2.3",
     "eslint-plugin-prettier": "^3.1.1",
     "eslint-plugin-react": "^7.16.0",
-    "html-webpack-plugin": "^3.2.0",
+    "html-webpack-plugin": "^4.2.1",
     "prettier": "^1.18.2",
     "react-hot-loader": "^4.12.15",
     "webpack": "^4.41.2",