Add popup to the Style Guide

Follows whatwg/html#5499.

Author: annevk

whatwg.org/style-guide

@@ -47,6 +47,7 @@
  <li>keepalive (though HTTP `<code>Keep-Alive</code>` [sic] header)</li>
  <li>metadata</li>
  <li>nonzero</li>
+ <li>popup</li>
  <li>queuing</li>
  <li>referrer (though HTTP `<code>Referer</code>` [sic] header)</li>
  <li>scrollbar</li>