title: Keep javascript bundle file beautify using uglify
body: <p>I'm using <code>rollup-plugin-uglify</code> with rollup
  I got development and production version of my JS bundle and I want on dev mode.
  keep my file beautify though I'm using uglify</p>
  
  <p>I tried to specified it as follows (on development build):</p>
  
  <pre><code>import uglify from 'rollup-plugin-uglify';
  
  export default {
      entry: 'myfile.js',
      dest: 'mybundle.js',
      plugins: [uglify({
          beautify: true,
          mangle: false,
          compress:false
      })]
  }
  </code></pre>
  
  <p>I'm using <code>rollup-plugin-uglify</code> with rollup
  I got development and production version of my js bundle and I want on development mode
  keep my file beautify though i'm using uglify</p>
  
  <p>But unfortunately the file output still uglified.</p>
  
  <p>Any idea?</p>