Presets for choosing base_learners to create stacked ensembles

[robertmartin8]

Let me preface this feature request by saying that I absolutely love this project, and would be happy to contribute however I can.

As implied by the title, I think it would be great if there were some preset options (other than Automated Ensemble Search) in creating stacked ensembles from the existing list of base_learners. Some suggestions:

• Create ensemble from the k best (as defined by any of the chosen metrics) base_learners
• Create ensemble from the k best base_learners per base_learner type. e.g the 3 best XGBoost classifiers, the 3 best RF classifiers, etc.
• Random ensemble creation