Fix gruntjs typings: Add grunt.config.merge method

gruntjs/gruntjs.d.ts

@@ -192,6 +192,12 @@ declare module grunt {
              */
             requires(prop: string, ...andProps: string[]): void
             requires(prop: string[], ...andProps: string[][]): void
+            
+            /**
+             * Recursively merges properties of the specified configObject into the current project configuration.
+             * You can use this method to append configuration options, targets, etc., to already defined tasks.
+             */
+            merge<T>(configObject: T): void;
         }
     }