Add DELETE operation

[yezyilomo]

The purpose of this is to delete nested objects i.e

{
  "add": [1,2,4],
  "delete": [8]
}

Just like REMOVE operation DELETE will be supporting __all__ value to i.e

{
  "add": [1,2,4],
  "delete": "__all__"
}

It will also be supporting allow_delete_all kwarg just like allow_remove_all

Another idea: Make __all__ value configurable through global django-restql settings