added CommandSet#list_commands, convenience method for repl exploration of command sets

lib/pry/command_set.rb

@@ -216,6 +216,11 @@ class Pry
     end
 
 
+    # @return [Array] The list of commands provided by the command set.
+    def list_commands
+      commands.keys
+    end
+
     private
     def define_default_commands