Allow indefinite duration for tone()

Bugfix: the `duration` parameter should be optional, per [`tone()`](https://www.arduino.cc/en/Reference/Tone) docs.

Author: urish

cores/esp8266/Tone.cpp

@@ -114,10 +114,14 @@ void noTone(uint8_t _pin) {
 }
 
 void t1IntHandler() {
-  if (toggle_counts[T1INDEX] > 0){
+  if (toggle_counts[T1INDEX] != 0){
     // toggle the pin
     digitalWrite(tone_pins[T1INDEX], toggle_counts[T1INDEX] % 2);
     toggle_counts[T1INDEX]--;
+    // handle the case of indefinite duration
+    if (toggle_counts[T1INDEX] < -2){
+      toggle_counts[T1INDEX] = -1;
+    }
   }else{
     disableTimer(T1INDEX);
     digitalWrite(tone_pins[T1INDEX], LOW);