Explicitly add _fill_optional parameter to baker.make and baker.prepare

[berinhard]

Recently I noticed that the _fill_optional argument isn't being displayed in Pycharm's helper dialog. Here's a print screen:

Expected behavior

The _fill_optional argument should be displayed the same way as the other parameters. The possible types are a list of strings with the field names or True to populate all boolean fields.

Actual behavior

The parameter is not being displayed.

Reproduction Steps

Try to use model-bakery with any modern Python IDE such as Pycharm or a configured VSCode.

Versions

Python: 3.8
Django: 2.2.
Model Bakery: 1.3.2

[berinhard]

Fixed by #264