title: pm2 Single Page Application flag --spa throws 'unknown option' error
body: <p>I am using <code>pm2</code> (5.1.2) and <code>serve</code> (13.0.2) to deploy a Node.js app. When I run the following:</p>
  <p><code>pm2 serve build 8080 --spa</code></p>
  <p>I receive the following error:</p>
  <p><code>error: unknown option '--spa'</code></p>
  <p>Is the flag deprecated? What could be causing this?</p>