Xsd-to-django-model

Added `DOC_PREPROCESSOR` setting.
Added support for `xs:unsignedInt` type.
More accurate XSD namespaces handling.
Warn when a column name exceeds 63 characters, do not fail.
Fixed JSON `attrs` inheritance in child models.
Fixed mapping of XSD types to model names with complex rules.
Do not emit model code for dependent models with `skip_code` set to `True`.
Fixed Python 3 / Django 2 compatibility in generated models code.

Added `skip_code` option to skip generated model code from output.

Fix a typo; allow `plain_index_fields` including `unique_fields`, which does not work yet because of https://code.djangoproject.com/ticket/24082

Fixed support for `add_fields` within `if_type` configuration branches.
Fixed support for `one_to_many` fields within `add_fields` configuration branches.

Fixed `class Meta` inheritance from abstract models; class-based `Meta.indexes` were not inherited from parent model if any `class Meta` was defined in the child model

* Fixed `related_name` clash when both `ForeignKey` and `ManyToManyField` to the same related model coexist.