(WIP) Separate sortable logic from Drag & Drop, and add generic Drag and Drop functionality

[nspaeth]

I need the functionality of cyclejs-sortable, along with other generic Drag and Drop functionality, so I started refactoring this to make cyclejs-sortable implemented via more generic Drag and Drop interfaces.

I'm making this WIP pull request as a kind of notice-of-intent, so that any issues can be discussed beforehand.

Modeling after react-dnd, I plan on adding:

• dragSource and dropTarget wrappers, to allow for dragging between containers
  • makeSortable would then internally be implemented by a combination of these interfaces
• A canDrop like interface
• Custom dragPreview(ghosts) rendering

[jvanbruegge]

Thanks for going forward with this! I wanted to do something like this for a long time, but as it is not really needed for my direct work I never had the time for it.