title: Use Webpack's DefinePlugin vars in Jest tests
body: <p>I'm pretty new in React, coming from Angular. I'm writing some tests for the code that's going to send request to an external endpoint. Obviously, I don't want to hardcode a real host in production code so I thought I could use webpack's <code>DefinePlugin</code> to keep this in a constant. </p>
  
  <p>How to configure Webpack and Jest together to use Webpack's capabilities? </p>