Add *args to the LEDBoard on(), off() and toggle() methods.

Implements #125

gpiozero/boards.py

@@ -173,16 +173,28 @@ class LEDBoard(LEDCollection):
         self._stop_blink()
         super(LEDBoard, self).close()
 
-    def on(self):
+    def on(self, *args):
         self._stop_blink()
+        if args:
+            for index in args:
+                self[index].on()
+        else:
             super(LEDBoard, self).on()
 
-    def off(self):
+    def off(self, *args):
         self._stop_blink()
+        if args:
+            for index in args:
+                self[index].off()
+        else:
             super(LEDBoard, self).off()
 
-    def toggle(self):
+    def toggle(self, *args):
         self._stop_blink()
+        if args:
+            for index in args:
+                self[index].toggle()
+        else:
             super(LEDBoard, self).toggle()
 
     def blink(