title: Webpack build very slow because of external libraries
body: <p>I try use webpack to my app, but it build my app for 60-100sec each time.
  How can i disable rebuild files from <code>node_modules/*</code> and  <code>bower_components/*</code> or build them for separate chunk (per module or shared).</p>
  
  <p>here is part of config:</p>
  
  <pre><code> {
      test: /\.js$/,
      exclude: /(node_modules|bower_components)/,
      loaders: ['ng-annotate', 'babel-loader']
   },
  </code></pre>
  
  <p>and</p>
  
  <pre><code>resolve: {
      moduleDirectories: ['node_modules', 'bower_components']
  },
  </code></pre>
  
  <p>How can i improve performance, by disabling rebuilding of libraries at each time?</p>