版本：
文件：
- https://cdn.bootcdn.net/ajax/libs/jquery.scrollfire/1.4.0/jquery.scrollfire.js
- https://cdn.bootcdn.net/ajax/libs/jquery.scrollfire/1.4.0/jquery.scrollfire.min.js
Allows useful callbacks to be fired upon elements scrolling into and out of view from both the top and bottom of the viewport/window.