Safari support

[LuisReinoso]

Hi, I really love this software ;)
But I note that this is not working on Safari Browser. Because active class is not setting on the DOM.

Could you give some advice that from where I should have to start to support this browser.

I have tried:

• install polyfills intersection-observer has you tell in readme.
• to check if it works I use how to detect browser support for IntersectionObserver to avoid load w3c/IntersectionObserver#296 (comment) but this return false also safari doesn't works
• and this one too. Don't work in Safari w3c/IntersectionObserver#282

I try to make it work on safari < 12.1. That doesn't have official support.

Thanks!