Use the 'isIntersecting' attribute as a fallback for zero threshold

[Zubrolet]

There is a bug in determining an intersection when the threshold option is missing or equal to 0(which is the default value according to specs).

Actual result:

1. Since in Chrome(Version 70.0.3538.110 and previous builds as well) intersection event for a target element fires only once with an intersectionRatio equal to 0 - wrapped component never get loaded;
2. As a solution to this problem, I would suggest using isIntersecting attribute which is true if the target intersects with the root. I have already tested that code and it gives the result for an entry: isIntersecting === true && intersectionRatio === 0;

Expected result:

1. Elements with missing or zero thresholds should be loaded correctly - it could be useful for lazy loading images;
2. The fix should be quite simple though -

   if (trackedElement &&
   (entry.isIntersecting || entry.intersectionRatio > 0)
   ) {
   trackedElement.visibilityHandler()
   }

[tazsingh]

This has since been fixed by #19. Thanks for the PR!