title: Webpack: Unknown argument: mode / configuration has an unknown property 'mode'
body: <p>getting crazy with this, really missing something....</p>
  
  <p>I have webpack 4.6.0, webpack-cli ^2.1.2, so the latest. </p>
  
  <p>Following the docs (<a href="https://webpack.js.org/concepts/mode/" rel="noreferrer">https://webpack.js.org/concepts/mode/</a>), want to use the mode to have to configs, one for production and one for development, but I get:</p>
  
  <p>configuration[0] has an unknown property 'mode'. These properties are valid:
     object { amd?, bail?, cache?, context?, dependencies?, devServer?, devtool?, entry, externals?, loader?, module?, name?, node?, output?, parallelism?, performance?, plugins?, profile?, recordsInputPath?, recordsOutputPath?, recordsPath?, resolve?, resolveLoader?, stats?, target?, watch?, watchOptions? }</p>
  
  <p>What am I missing :O????? </p>
  
  <pre><code>module.exports = [
    merge(base, {
      mode: 'development',
      output: {
        path: path.resolve(__dirname, './public/assets/development'),
      },
    }),
    merge(base, {
      mode: 'production',
      output: {
        path: path.resolve(__dirname, './public/assets/production'),
        filename: '[name].bundle.js',
      },
    }),
  ]
  </code></pre>