Better class feature, and better API

• In respond of request #19, now you can fill the data-scroll attributes with as many classes as you need, so it's compatible with any CSS framework.
  If you leave it empty, the class is-outside will be used by default.

• Better API: now you can test if an element is isInsideViewport, isRepeating or get repeatingCount.
  Go check the API section in the documentation.

New `reverse` option

The default behaviour of this library is adding a class when the element is outside of the viewport, but sometimes, you may need the opposite behaviour, i.e. adding a class when the element is inside the viewport.

So here is a new attribute data-scroll-reverse: Instead of adding the class when the element is outside of the viewport, reverse this behaviour and add the class when the element is inside the viewport, is-inside class name as default if data-scroll is empty.

In this case, you will probably need to add the class to your element as default in your code to avoid JavaScript dependency.

Thanks @CFT-Chris for the pull request 👍

Fix for strict mode

No arguments.callee for strict mode.

Initial version!

Features:

• A class to be added when element goes outside of viewport, is-outside if empty
• Infinite repeating, several times or once
• An offset to be taken into account fromtop and bottom edges of the viewport
• Custom prefix for html attributes
• Asynchronous initialization
• Hooks enable
• State properties

and documentation.