Snapping Sidescroller

The `snap` extension ensures that the Sidescroller’s child elements are always aligned flush with the Overthrow container when scrolling ends.

Snapping is added to a Sidescroller by specifying snapScroll: true in the intialization options: overthrow.sidescroller( document.querySelectorAll( ".overthrow-enabled .sidescroll-nextprev" ), {snapScroll: true });