title: React-Cesium webpack config loader issue
body: <p>I'm new to playing with Webpack configuration and attempting to incorporate <a href="http://httphttps://github.com/rot1024/cesium-react" rel="nofollow noreferrer">React-Cesium</a> into my project. I'm following along with the Github instructions for installing React-Cesium and running into this error: 
  <a href="https://i.stack.imgur.com/Ise7Z.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Ise7Z.png" alt="enter image description here"></a> </p>
  
  <p>Here is my webpack.config.js file after adding what the documentation told me to:</p>
  
  <pre><code>'use strict';
  
  const UglifyJSPlugin = require('uglifyjs-webpack-plugin');
  
  module.exports = {
    module: {
      externals: {
        cesium: "Cesium"
      },
      output: {
         publicPath: "/"
           // ...
      },
      alias: {
        cesiumSource: "cesium",
        cesium: "cesium/Cesium"
      },
      rules: [
        {
          test: /\.pug$/,
          use: [
            'pug-loader?self'
          ]
        },
        {
          test: /\.css$/,
          use: [
            'style-loader',
            'css-loader'
          ]
        },
  
      ]
    },
    plugins: [
      new UglifyJSPlugin()
      new CopyPlugin([
       {
         from: `node_modules/cesium/Build/Cesium${prod ? "" : "Unminified"}`,
         to: "cesium"
       }
     ]),
     new HtmlIncludeAssetsPlugin({
       append: false,
       assets: [
         "cesium/Widgets/widgets.css",
         "cesium/Cesium.js"
       ]
     }),
     new webpack.DefinePlugin({
       "process.env": {
         NODE_ENV: JSON.stringify(opts &amp;&amp; prod ? "production" : "development"),
         CESIUM_BASE_URL: JSON.stringify("/cesium")
       }
     })
  
   }
    ],
  
  };
  </code></pre>
  
  <p>The documentation is a little ambiguous and I'm not sure what's wrong. The documentation says to install <strong>npm i html-include-assets-plugin --save-dev</strong>. That command doesn't work, I get a 404 error when attempting to install it.</p>
  
  <p>Is that what is causing the loader error? If not, then what is?</p>