Add NeoBusChannel constructor to NeoPixelBrightnessBus (#491)

src/NeoPixelBrightnessBus.h

@@ -70,6 +70,12 @@ public:
         _brightness(255)
     {
     }
+    
+    NeoPixelBrightnessBus(uint16_t countPixels, uint8_t pin, NeoBusChannel channel) :
+        NeoPixelBus<T_COLOR_FEATURE, T_METHOD>(countPixels, pin, channel),
+        _brightness(255)
+    {
+    }
 
     NeoPixelBrightnessBus(uint16_t countPixels, uint8_t pinClock, uint8_t pinData) :
         NeoPixelBus<T_COLOR_FEATURE, T_METHOD>(countPixels, pinClock, pinData),