title: Tried to rebuild dist folder but fail
body: <p>I found a great library, which can make draft.js even better.</p>
  
  <p><a href="https://github.com/jpuri/react-draft-wysiwyg" rel="nofollow noreferrer">https://github.com/jpuri/react-draft-wysiwyg</a></p>
  
  <p>I create a project, and npm install this library.</p>
  
  <p>But inside this library's folder, I tried to rebuild the dist folder, 
  cause I customize some setting in my project.</p>
  
  <p>When I run <strong>npm run build</strong>, it gave me some error:</p>
  
  <pre><code>ERROR in ./src/Editor/index.js
  Module build failed: SyntaxError: Unexpected token (41:19)
  
    40 | export default class WysiwygEditor extends Component {
  &gt; 41 |   static propTypes = {
       |                    ^
  </code></pre>
  
  <p>I already create node_modules &amp; config/webpack.config.js inside this library's folder.</p>
  
  <p>why is that happen?</p>
  
  <p><br>
  <hr>
  Added my <strong>webpack.config.js</strong> at following:</p>
  
  <pre><code>const path = require('path');
  const webpack = require('webpack');
  const ExtractTextPlugin = require('extract-text-webpack-plugin');
  const autoprefixer = require('autoprefixer');
  const precss = require('precss');
  
  module.exports = {
      devtool: 'source-map',
      entry: [
          './src/index',
      ],
      output: {
          path: path.join(__dirname, '../dist'),
          filename: 'react-draft-wysiwyg.js',
          library: 'reactDraftWysiwyg',
          libraryTarget: 'umd',
      },
      externals: {
          react: 'react',
          immutable: 'immutable',
          'react-dom': 'react-dom',
          'draft-js': 'draft-js',
      },
      plugins: [
          new webpack.DefinePlugin({
              'process.env': {
                  NODE_ENV: JSON.stringify('production'),
              },
          }),
          new webpack.optimize.UglifyJsPlugin({
              compressor: {
                  warnings: false,
              },
          }),
          new ExtractTextPlugin('react-draft-wysiwyg.css'),
          new webpack.LoaderOptionsPlugin({
              options: {
                  postcss: [autoprefixer, precss],
              },
          })
      ],
      module: {
          loaders: [
              { test: /\.js$/, loader: 'babel-loader', exclude: /immutable\.js$|draftjs-utils\.js$/ },
              {
                  test: /\.css$/,
                  loader: ExtractTextPlugin.extract({
                      fallback: 'style-loader',
                      use: 'css-loader?modules&amp;importLoaders=1&amp;localIdentName=[local]!postcss-loader',
                  }),
              },
              { test: /\.(png|jpg)$/, loader: 'url-loader?limit=8192' },
              {
                  test: /\.svg(\?v=\d+\.\d+\.\d+)?$/,
                  loader: 'url-loader?limit=10000&amp;mimetype=image/svg+xml',
              },
          ],
      },
      resolve: {
          extensions: ['.js', '.json'],
      },
  };
  </code></pre>