Add an option to use only vertical element position to determine its view-ability

[gajus]

blazy/blazy.js

Lines 164 to 170 in b0cc033

	function elementInView(ele) {
	var rect = ele.getBoundingClientRect();
	return (
	// Intersection
	rect.right >= _viewport.left && rect.bottom >= _viewport.top && rect.left <= _viewport.right && rect.top <= _viewport.bottom
	);
	}

The current implementation is checking whether element is in a viewport vertically and horizontally. There are cases (e.g. a carousel) when the intention is to load all elements that are in viewport vertically, disregarding horizontally.

[gajus]

@dinbror Would you like me to raise a PR for this feature?