Added new feature 'Multiplier' (zoom by click)

[mafftor]

I have added a feature which works by click. Click on image and zoom it easy.

Initialize in this way if you want to disable the multiplier feature

wheelzoom(document.querySelector('img.zoom'), {maxMultiplier: 0});

or set the max of depth

wheelzoom(document.querySelector('img.zoom'), {maxMultiplier: 5});