title: Custom webpack config alias not working in ionic 2
body: <p>I am using environment alias in my custom config like this:
  package.json</p>
  
  <pre><code> "config": {
      "ionic_bundler": "webpack",
      "ionic_webpack": "./config/webpack-extension.config.js"
    }
  </code></pre>
  
  <p>webpack-extension.config.js</p>
  
  <pre><code>/*
  inspired by:
      https://github.com/driftyco/ionic-app-scripts/pull/683#issuecomment-287401855
  */
  
  let path = require('path')
  let useDefaultConfig = require('@ionic/app-scripts/config/webpack.config.js')
  
  module.exports = function () {
  
  useDefaultConfig.resolve.alias = {
      "@app/env": path.resolve(__dirname, '../src/environments/env.' + process.env.IONIC_ENV + '.ts'),
  };
  
  return useDefaultConfig;
  }
  </code></pre>
  
  <p>environment.ts</p>
  
  <pre><code>import {
  
  isDEV,
  API_HOST,
  ...etc
  
  } from '@app/env'
  </code></pre>
  
  <p>But I am getting error:</p>
  
  <pre><code>Typescript Error
  Cannot find module '@app/env'.
  </code></pre>