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.
`__init__` shouldn't initialize attributes that `fit` is going to fill in. That's `fit`'s responsibility. (Ideally, checking whether an estimator has been fitted is checking whether it has attributes ending with `_`.)

Also, we don't like private attributes. Can this be done without additional attributes?