版本：
文件：
- https://cdn.bootcdn.net/ajax/libs/jQuery-viewport-checker/1.8.8/jquery.viewportchecker.min.js
- https://cdn.bootcdn.net/ajax/libs/jQuery-viewport-checker/1.8.8/jquery.viewportchecker.min.js.map
Little script that detects if an element is in the viewport and adds a class to it.