Export default modelgen hooks

[atzedus]

When customizing modelgen plugin with FieldHook for example, then we need to copy whole code of defaultFieldMutateHook to our newly created hook. If we forget to do this, then we loose @gotag and @gofield functionality.
With exported code we can simply call it in our custom hook before or after custom code.

I have:

• Updated any relevant documentation (see docs)

[coveralls]

Coverage decreased (-0.003%) to 75.669% when pulling c2ef8d4 on atzedus:exported-modelgen-hooks into 6b8c6ee on 99designs:master.