API to customize the list of TYPE_MAPPING

Nobody has claimed this yet.

I use sqlalchemy_utils which provide ChoiceType field (and many others).

I'm able to create a new Marshmallow field (Field) to handle this new field with proper `_serialize/_deserialize` functions but instead to override the attributes one by one in each schema I defined I want to update the list type_mapping in ModelConverter.

I didn't find how to access it properly, ModelConverter accepts `schema_cls` attribute at init
https://github.com/marshmallow-code/marshmallow-sqlalchemy/blob/dev/marshmallow_sqlalchemy/convert.py#L76
but I'm not able to figure out how to use it properly with all these meta classes ;)

So for now I rely on a monkey patch:

```
ModelConverter.SQLA_TYPE_MAPPING.update({
ChoiceType: ChoiceTypeSchema
})
```

PS: I use flask-marshmallow in my app.

Start in marshmallow_sqlalchemy/convert.py at ModelConverter and SQLA_TYPE_MAPPING, then review the issue discussion for the intended customization boundary. Done means users can register a custom SQLAlchemy field mapping through a supported API instead of monkey-patching the class mapping.

Written by the indexing model from the issue text.