Feature: String format type mapping

[dan3988]

Adds the ability to specify custom types & a serialize/deserialize function for properties based on the "format" property.

Implementation of #719

      swagger_dart_code_generator:
        options:
          input_folder: "input_folder/"
          output_folder: "lib/swagger_generated_code/"
          import_paths:
            - "package:uuid/uuid.dart"
          scalars:
            uuid:
              type: Uuid
              deserialize: Uuid.parse
              # optional - default is toString()
              serialize: myCustomUuidSerializeFunction