insert many and ids
Currently if you are using Postgresql you can write:

```python
query = User.insert_many([{...}, {...}, {...}]).return_id_list()
ids_inserted = query.execute()
```

It's admittedly a weird API and not very flexible. In 3.0a it is the default to return the id list when the database supports the returning clause.
Hi!
Is it possible to insert many records and get them id list?
Or maybe id of the last inserted. Or get the next identity value?
In one transaction. In postgresql.
Would be awesome!