Return dict from scalar
I have some analytical queries that look like this:
```python
import peewee as pw

query = Model.select(
    pw.fn.COUNT(Model.field).alias('name')
)
```
and because I am usually passing result further as dict, I have to do it like this:
```python
import peewee_async

manager = peewee_async.Manager()

result = (await manager.execute(query.dicts())[0]
```
What I'd like to have is and option to write something like this:
```python
result = await manager.execute(query.scalar(as_dict=True))
```

I'd be happy to contribute the PR if this is a valid request