Add a brightness API for the rainbow effects

src/Kaleidoscope-LEDEffect-Rainbow.cpp

@@ -19,6 +19,10 @@ void LEDRainbowEffect_::update(void) {
   LEDControl.set_all_leds_to(rainbow);
 }
 
+void LEDRainbowEffect_::brightness(byte brightness) {
+   rainbow.value=brighness;
+}
+
 LEDRainbowEffect_ LEDRainbowEffect;
 
 // ---------
@@ -47,4 +51,8 @@ void LEDRainbowWaveEffect_::update(void) {
   }
 }
 
+void LEDRainbowWaveEffect_::brightness(byte brightness) {
+   rainbow.value=brighness;
+}
+
 LEDRainbowWaveEffect_ LEDRainbowWaveEffect;

src/Kaleidoscope-LEDEffect-Rainbow.h

@@ -7,6 +7,7 @@ class LEDRainbowEffect_ : public LEDMode {
  public:
   LEDRainbowEffect_(void);
 
+  void brightness(byte);
   void update(void) final;
 
  private:
@@ -27,6 +28,7 @@ class LEDRainbowWaveEffect_ : public LEDMode {
  public:
   LEDRainbowWaveEffect_(void);
 
+  void brightness(byte);
   void update(void) final;
 
  private: