title: react-dnd : Check if dragged source is before of after original place in ordered list
body: <p>I'm using react-dnd.  I want to know if the currently hovered target is before or after the dragged source in an ordered list of dnd dom objects and pass this to the dom object as a css class. I want to be able to compare props between source and target for my own custom functions. </p>
  
  <p>How can I create custom source-target-relation functions (like isOver, canDrop, etc)? I want to create a isBeforeDraggedSource() function.</p>