build(webpack): upgrade to webpack v4 and add build script

devDependencies:
* webpack@4.19.0
* webpack-cli@3.1.0

Author: remarkablemark

package.json

@@ -5,19 +5,20 @@
   "author": "Mark <mark@remarkablemark.org>",
   "main": "index.js",
   "scripts": {
-    "build:unmin": "NODE_ENV=development webpack index.js dist/html-react-parser.js",
-    "build:min": "NODE_ENV=production webpack -p index.js dist/html-react-parser.min.js",
+    "build": "npm run clean && npm run build:min && npm run build:unmin",
+    "build:min": "NODE_ENV=production webpack -o dist/html-react-parser.min.js",
+    "build:unmin": "NODE_ENV=development webpack -o dist/html-react-parser.js",
     "clean": "rm -rf dist",
     "commitmsg": "commitlint -e $GIT_PARAMS",
     "cover": "istanbul cover _mocha -- -R spec \"test/**/*\"",
     "coveralls": "cat coverage/lcov.info | coveralls",
     "lint": "eslint --ignore-path .gitignore .",
     "lint:fix": "npm run lint -- --fix",
     "precommit": "lint-staged",
-    "prepublishOnly": "npm run clean && npm run build:unmin && npm run build:min",
+    "prepublishOnly": "npm run build",
     "release": "standard-version --no-verify",
     "test": "mocha",
-    "test:benchmark": "node ./test/benchmark.js"
+    "test:benchmark": "node test/benchmark.js"
   },
   "repository": {
     "type": "git",
@@ -53,7 +54,8 @@
     "react": "^16",
     "react-dom": "^16",
     "standard-version": "^4.3.0",
-    "webpack": "^3.11.0"
+    "webpack": "^4.19.0",
+    "webpack-cli": "^3.1.0"
   },
   "peerDependencies": {
     "react": "^0.14 || ^15 || ^16"

webpack.config.js

@@ -1,31 +1,21 @@
-'use strict';
+var path = require('path');
 
 /**
- * Module dependencies.
- */
-var webpack = require('webpack');
-
-/**
- * Export webpack configuration.
+ * Webpack configuration.
  */
 module.exports = {
+  entry: path.resolve(__dirname, 'index.js'),
   output: {
     library: 'HTMLReactParser',
     libraryTarget: 'umd'
   },
   externals: {
     react: {
-      root: 'React',
-      commonjs2: 'react',
+      amd: 'react',
       commonjs: 'react',
-      amd: 'react'
+      commonjs2: 'react',
+      root: 'React'
     }
   },
-  plugins: [
-    new webpack.optimize.ModuleConcatenationPlugin(),
-    new webpack.optimize.OccurrenceOrderPlugin(),
-    new webpack.DefinePlugin({
-      'process.env.NODE_ENV': JSON.stringify(process.env.NODE_ENV)
-    })
-  ]
+  mode: process.env.NODE_ENV
 };