on_conflict support in bulk_create
I'm going to pass on implementing this as part of `bulk_create()` since this can be easily handled by:

```python
Model.insert_many([....]).on_conflict_ignore()
```

The problem arises w/postgres where the insert cursor returns IDs only for the rows that are created, which can't be mapped back to the actual objects that were inserted.
Please provide mechanism to ignore conflicts when calling bulk_create.

My target database is PostgreSQL.

At th moment I am handrolling my own query.