A function that waits for a specific element to load before running a specified callback. portated from this GitHub gist: https://gist.github.com/BrockA/2625891#file-waitforkeyelements-js
This script should not be not be installed directly. It is a library for other scripts to include with the meta directive // @require https://update.greasyfork.org/scripts/476008/1255570/waitForKeyElements%20gist%20port.js
