Add option to cause images to fill the slider

Adding the “zoom = true” option will cause images to be zoomed &
centred in the slider div so that they fill the slider without being
distorted
Set zoom function to work with divs containing imgs

If zoom is set to true, children imgs of div slides will also be
expanded in the same way that imgs are
Remove test console.logs
Move zooming of images to initial load

Zooming actually only needs to be done once since the slider preserves
its aspect ratio
Added public facing resize function

Allow the slider to be resized using javascript after it’s loaded.