Expose classes_ attribute on classification Pipeline objects

[rtanase]

Is your feature request related to a problem? Please describe.
If a Pipeline is being used for a classification problem, it would be great to include the determined classes during training so that predict_proba output can be interpreted.

Describe the solution you'd like
I would like the Pipeline object to expose a classes_ attribute, just like a classifier would do if used on its own (without being wrapped in a Pipeline).

Describe alternatives you've considered
When used outside of a Pipeline, the BasePredictor class ensures that classifiers expose this attribute.

Additional context
Add any other context or screenshots about the feature request here.