title: create-react-app use custom service worker without ejecting
body: <p>I have an existing app I'm trying to convert to use React. I've copied its functionality with a brand new <code>create-react-app</code> one (using <code>react-scripts</code> 1.0.13).</p>
  
  <p>I want to use the existing service worker I have. I've noticed CRA creates its own service worker; there is code in the webpack config (using <code>SWPrecacheWebpackPlugin</code>) that creates an unbundled module, <code>service-worker.js</code>. All other JS modules are bundled together.</p>
  
  <p>From what I understand, I can't just copy my existing service worker <code>existing-service-worker.js</code> and try to import that, as all JS modules are bundled together.</p>
  
  <p><strong>I don't want to eject.</strong></p>
  
  <p>I've forked <code>create-react-app</code> in order to customise <code>react-scripts</code> and use different logic in the webpack config, which will allow me to use my existing service worker instead of the one it creates with <code>SWPrecacheWebpackPlugin</code>... However, I don't know how to do this. This is my first time using React and webpack. </p>
  
  <p>Can someone point me in the right direction, and help me use my existing service worker in React, without ejecting?</p>