Added tone function

Added tone function.
Syntax:
radio.tone(x, y, z);

x = length in mS
y = Amplitude in dB, -30 to 0
z = Frequency in Hertz. (10 to 20000)

Note. After this command the audio is set to Unmute.

src/TEF6686.h

@@ -63,6 +63,7 @@ class TEF6686 {
     void setStereoLevel(uint16_t start);
     void setUnMute();
     void setVolume(int16_t volume);
+	void tone(uint16_t time, int16_t amplitude, uint16_t frequency);
     uint8_t af_counter;
 
   private: