dict_vectorizer: added inverse_onehot to original feature names + updatable vectorizer dict

added features to:
- allow inverse onehot names to original names.
- allow to update current dict when using fit_transform multiple times and stay compatible with previous encoding.
- this initialization is required in the case _transform is used in update mode (update_dict=True). It will re-use the current values of feature_names_, vocabulary_, _onehot_dict. So the first time they should already exists to avoid multiple inelegant checks eack time _transform is called with update_dict=True. 

- would you prefer onehot_dict_ instead of _onehot_dict ?