added led.enable (#294)

* added led.enable

* fixed simulator support for led.enable

docs/reference/led/enable.md

@@ -0,0 +1,33 @@
+# Enable
+
+Turns the LED screen on and off
+
+```sig
+led.enable(false);
+```
+
+### Parameters
+
+* ``on`` is a [boolean](/reference/types/boolean) that defines the on/off state of the screen
+
+### Example: Turning off the screen
+
+This program turns off the screen when pressing button ``B``
+
+```typescript
+input.onButtonPressed(Button.B, () => {
+    led.enable(false)
+});
+```
+
+### Pins: P3, P4, P6, P7, P9, P10
+
+These pins are coupled to the LED matrix display, and also it’s associated ambient light sensing mode.
+To disable the display driver feature (which will automatically disable the light sensing feature) call the DAL function ``led.enable(false)``. 
+To turn the display driver back on again later, call ``led.enable(true)``.
+
+More information at http://tech.microbit.org/hardware/edgeconnector_ds/ .
+
+### See also
+
+[unplot](/reference/led/unplot), [point](/reference/led/point), [LED screen](/device/screen)