Experiment on_winner_choose hook #571

Hey folks.

In our application, we have to know when a particular experiment's winner was chosen.

There are already implemented integration point such as:

• on_trial
• on_trial_choose
• on_trial_complete
• on_before_experiment_reset
• on_experiment_reset
• on_experiment_delete

I propose to implement a hook like:

• on_winner_choose

If you have a solution which doesn't require the implementation of this hook please let me know.

If a solution doesn't exist, please let me know if you are interested in this functionality or not. It looks like I will add it anyway, but in prospect, it can be part of the library.