Add "container" setting, to allow attaching AOS to any scrollable element

[m-kutnik]

Related Issue

Issue: #223

Your solution

• Added container to AOS options (default: window).
• Variable container is pointing to HTML Element, or to Window if querySelector returns null.
• EventListener is passing container to handleScroll() as a second argument
• handleScroll() evaluates scroll distance by using Element.scrollTop or Window.pageYOffset

Defining scrollable element

AOS.init({
  container: ".scrollable-div"
})

How Has This Been Tested?

• By running included tests

In own projects, tested on:

• Desktop: Firefox 62, Chrome 71, Edge 42
• Android: Chrome, Firefox