Update plugin.elementhelper.php

Added native modx caching mechanism, like this the plugin can be run in a production environment (if desired) without slowing down the page.

The changes speed the plugin up significantly (~60 files takes about 1.8 seconds on every page rendering, with new code it's down to 0.04s and if something is changed around 1s)

also included here is the change to check the usergroup from earlier commits

no idea if you think this is helpful, just wanted to share and if you like, feel free to take whatever you think makes sense for the next release! Love this plugin =)