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

[gajus]

https://github.com/dinbror/blazy/blob/b0cc033a0c968094bf15987368b55231878ae887/blazy.js#L164-L170

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?