Added special expressions to boolean field
Going to pass for now, thanks.
Added an expression so that in the request instead of:
```python
Model.select().where(Model.boolean_field == True)
````
To write:
```python
Model.select().where(Model.boolean_field.is_true())
````

The same expression already exists:
https://github.com/alteralt/peewee/blob/b8c629bc100c4886c5b87b6eb62cce66c84a0d9c/peewee.py#L1197