make the abstract item disappeared

[yoheimiyamoto]

• I'm submitting a ...
  [ ] bug report
  [ ] feature request
  [○] question about the decisions made in the repository

• Do you want to request a feature or report a bug?
  a feature

• What is the current behavior?
  The dragging item and abstract item are applied same css ".dnd-sortable-drag".

• What is the expected behavior?
  I want to change the opacity of abstract item to 0.0 like following sample.
  http://react-dnd.github.io/react-dnd/examples-sortable-simple.html

• What is the motivation / use case for changing the behavior?
  improving UI.

• Please tell us about your environment:

  • Angular version: 5.2.0
  • Browser: [ Chrome ]