Support standard threshold option

[sntran]

Can take either single value or an array of them, just as the Intersection Observer.

Also, switch to check entry.isIntersecting instead of comparing
against the threshold.

[mciastek]

What is a use case of updated threshold option? The idea behind sal.js is to have an abstraction upon Intersection Observer API, which allows you to easily animate elements on scroll. Adding more options and giving full access to Intersection Observer options is not a goal of this library.

[sntran]

This new threshold option is actually related to #11 , in which I need to know how much of intersection that occurs. Think parallax effect. Having a single number for threshold means I only know once when the intersection occurs.

I'm not adding more option or giving full access to IO here. Just make one existing option more usable.

[mciastek]

Think parallax effect. Having a single number for threshold means I only know once when the intersection occurs.

I think that sal.js is proper library to make parallax effect. Sal it's fine for simple in-out animation on scroll position. To be honest, I don't see a good use case for extended threshold option, and I think it can introduce some misunderstandings.

[sntran]

Sure. I'll close this. However, I would like to see an example of parallax with sal.js.