fix(perf): less extra watcher per draggable/droppable if data-drag/data-drop is an interpolated string
fix(perf): less extra watcher per draggable/droppable if data-drag/data-drop is an interpolated string

Closes #168

timonsk/angular-dragdrop,seawenzhu/angular-dragdrop,timonsk/angular-dragdrop,youthoughts/angular-dragdrop,mauricedw22/angular-dragdrop,SteveWoodis/angular-dragdrop,SteveWoodis/angular-dragdrop,roni5/canvas,seawenzhu/angular-dragdrop,youthoughts/angular-dragdrop,nishant8BITS/angular-dragdrop,nishant8BITS/angular-dragdrop,mauricedw22/angular-dragdrop,JsonChiu/angular-dragdrop,JsonChiu/angular-dragdrop,codef0rmer/angular-dragdrop,roni5/canvas