add feature_names to accessor functions

Author: ablaom

src/accessor_functions.jl

@@ -93,6 +93,25 @@ LearnAPI.strip(LearnAPI.strip(model)) == LearnAPI.strip(model)
 """
 LearnAPI.strip(model) = model
 
+"""
+    LearnAPI.feature_names(model)
+
+Return the names of features encountered when fitting or updating some `learner` to obtain
+`model`.
+
+The value returned value is a vector of symbols.
+
+This method is implemented if `:(LearnAPI.feature_names) in LearnAPI.functions(learner)`.
+
+See also [`fit`](@ref).
+
+# New implementations
+
+$(DOC_IMPLEMENTED_METHODS(":(LearnAPI.feature_names)")).
+
+"""
+function feature_names end
+
 """
     LearnAPI.feature_importances(model)
 
@@ -291,14 +310,14 @@ $(DOC_IMPLEMENTED_METHODS(":(LearnAPI.training_labels)")).
 """
 function training_labels end
 
-
 # :extras intentionally excluded:
 const ACCESSOR_FUNCTIONS_WITHOUT_EXTRAS = (
     learner,
     coefficients,
     intercept,
     tree,
     trees,
+    feature_names,
     feature_importances,
     training_labels,
     training_losses,