text: Title: Custom Scorer for CV inside plot_learning_curve
  Body: Hello,
  
  I am using cross-validation with a particular metric, Kappa score, rather than the standard accuracy metric.
  
  `cross_val_score(clf, x_train, y_train, scoring=kappa_scorer, cv=kf, n_jobs=-1)`
  
  I would like to to set the CV done inside the `plot_learning_curve` method for each set of `train_sizes` to use the Kappa Scorer and not the accuracy score. I would also like to use the Kappa Scorer to evaluate the models performance for the training set. Is there any way to set this in the `plot_learning_curve` method ?