Possibility to merge related/similar items

[ruvan83]

Change each [ ] to [x] when you have done it.

• My issue title starts with [usage question]
• I have read the README, especially the 'classic blunders' section
• I have looked at the demos to see if my use case has obvious examples.
• I have searched the ng2-dragula issues, including closed issues.
• I have browsed the issues labeled "future reference" for problems that have been solved before.

Hello, I have an application based on this open source project which uses ng2-dragula for drag and drop items, would like to add the possibility to merge or group cards that have similar or related content together, so in this screenshot, I want to merge say the first card in Add column with the first in Less column.

and code for the component is here:https://github.com/AndreiSasu/portalizer.org/blob/master/portalizer/src/main/webapp/app/retro/board-details/board-details.component.ts#L110