[R-package] Add best score (#721)

* Try to store best scores in R package Microsoft/LightGBM#686

* Add binding for best_score (unlock environment)

* Fix double comma

* Add best_score to lgb.Booster

R-package/R/callback.R

@@ -9,6 +9,7 @@ CB_ENV <- R6Class(
     eval_list = list(),
     eval_err_list = list(),
     best_iter = -1,
+    best_score = -1,
     met_early_stop = FALSE
   )
 )
@@ -350,6 +351,7 @@ cb.early.stop <- function(stopping_rounds, verbose = TRUE) {
           
           # Check if model is not null
           if (!is.null(env$model)) {
+            env$model$best_score <- best_score[i]
             env$model$best_iter <- best_iter[i]
           }
           

R-package/R/lgb.Booster.R

@@ -4,6 +4,7 @@ Booster <- R6Class(
   public = list(
     
     best_iter = -1,
+    best_score = -1,
     record_evals = list(),
     
     # Finalize will free up the handles