title: How to remove old files from the build dir when webpack --watch?
body: <p>When my <code>webpack.config.js</code> is set to watch my source files, and the output files contain a hash, then each time the build successfully completes an entirely new set of built files exists. This quickly fills up the build dir with cruft!</p>
  
  <p>How do I make <code>webpack</code> delete the old files on each build?</p>
  
  <pre><code>module.exports = {
    ...
    watch: true,
    output: {
      filename: '[name]-[hash:8].js'
    }
    ...
  }
  </code></pre>
  
  <hr>
  
  <p>I recognize I could use <code>webpack-dev-server</code> to build in memory but that doesn't fit my current build process.</p>