added led.enable (#294)

* added led.enable

* fixed simulator support for led.enable

libs/core/shims.d.ts

@@ -488,12 +488,19 @@ declare namespace led {
 
     /**
      * Sets the display mode between black and white and greyscale for rendering LEDs.
-     * @param mode TODO
+     * @param mode mode the display mode in which the screen operates
      */
     //% weight=1 help=led/set-display-mode
-    //% parts="ledmatrix" shim=led::setDisplayMode
+    //% parts="ledmatrix" advanced=true shim=led::setDisplayMode
     function setDisplayMode(mode: DisplayMode): void;
 
+    /**
+     * Turns on or off the display    
+     */
+    //% help=led/enable blockId=device_led_enable icon="\uf04d"
+    //% advanced=true parts="ledmatrix" shim=led::enable
+    function enable(on: boolean): void;
+
     /**
      * Takes a screenshot of the LED screen and returns an image.
      */