title: How to load an external javascript file asynchronously within popup.html?
body: <p>I've got everything working, but after adding an external javascript file (which is, by the way, only a couple of short lines in length), it delays loading of the popup.html.</p>
  
  <p>This delay is annoying, and I think by asynchronously loading the javascript file, then it will get rid of this lag.</p>
  
  <p>The file is written in popup.html like this:</p>
  
  <p><code>&lt;script src="https://domain.com/myexternalscript.js"&gt;&lt;/script&gt;</code></p>
  
  <p>I'm not sure how to asynchronously load this file. So how can I do this?</p>