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.
I thought it was ok as long as it is a private method. Then do you want this parameter to be fixed at initialization ? In this case it would change the possible solutions for my answer above concerning initialization and reset of the different dicts in __init__ and fit ?