title: Pre build es6 npm modules with webpack in production config with uglifyjs
body: <p>In production, I use webpack config with UglifyJsPlugin.
  As you know there are some npm modules with es6 syntaxes. During deploy on production I am getting error:</p>
  
  <blockquote>
    <p>ERROR in bundle.js from UglifyJs Unexpected token name «i», expected
    punc «;» [./~/joi/lib/index.js:167,0]</p>
  </blockquote>
  
  <p>This is because joi module uses es6 syntax (for (let i in etc..)), but uglify can't handle it.
  I've solved problem, precompile several modules (only who use es6) with babel-cli util</p>
  
  <pre><code>babel src lib
  </code></pre>
  
  <p>and replace old folder with new one. But it is noncense =). How can I handle array of node modules (not all!, only specified) with webpack config? Thank you!</p>