Passing max_length to CharField when used in ArrayField
Please post questions to stackoverflow.

```python
some_field = ArrayField(CharField, field_kwargs={'max_length': 20}, null=True)
```

http://docs.peewee-orm.com/en/latest/peewee/playhouse.html#ArrayField
How would I pass max_length here:

```
some_field = ArrayField(peewee.CharField(max_length=20), null=True)
```

When I try to do that I get:

```
TypeError: 'CharField' object is not callable
```