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
此脚本不应直接安装。它是供其他脚本使用的外部库，要使用该库请加入元指令 // @require https://update.greasyfork.org/scripts/476008/1255570/waitForKeyElements%20gist%20port.js
