feat(collection): $addUnique for MongoDB $addToSet support (from yoneal PR #252).

Fixed #140
Opted for $addUnique because it is much simpler than $addToSet.
Below is an example usage:

```
{
  "persons": {
    "$addUnique": ["me", "you"]
  },
  "colors": {
    "$addUnique": "pink"
  }
}