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.
That's the first time I contribute to sklearn but it seems to me this allows to avoid duplicate code and it is a kind of private method (_) so does not alter common API on fit and fit_transform ?