Allow object type for the i18n property

[villesiltala]

The documentation describes master linking with the following specifications:

• i18n (object) (Optional) Custom localization information is stored in the property. It must contain an object with the following properties...
  The validation does not follow the documentation as it only approves an associative array as the "i18n" value. See:
  https://github.com/devgeniem/wp-geniem-importer/blob/master/src/Post.php#L573

Fix this by allowing the "i18n" value to be passed as either an associative array or an object.

To further improve type handling, a breaking change would be to create a class for the "i18n" and to add the class as the type of the "i18n" property.