Add RepeatTimer example (#216)

libraries/ESP32/examples/Timer/RepeatTimer/RepeatTimer.ino

+/*
+ Repeat timer example
+
+ This example shows how to use hardware timer in ESP32. The timer calls onTimer
+ function every second. The timer can be stopped with button attached to PIN 0
+ (IO0).
+
+ This example code is in the public domain.
+ */
+
+// Stop button is attached to PIN 0 (IO0)
+#define BTN_STOP_ALARM    0
+
+hw_timer_t * timer = NULL;
+
+void onTimer(){
+  static unsigned int counter = 1;
+
+  Serial.print("onTimer no. ");
+  Serial.print(counter);
+  Serial.print(" at ");
+  Serial.print(millis());
+  Serial.println(" ms");
+
+  counter++;
+}
+
+void setup() {
+  Serial.begin(115200);
+
+  // Set BTN_STOP_ALARM to input mode
+  pinMode(BTN_STOP_ALARM, INPUT);
+
+  // Use 1st timer of 4 (counted from zero).
+  // Set 80 divider for prescaler (see ESP32 Technical Reference Manual for more
+  // info).
+  timer = timerBegin(0, 80, true);
+
+  // Attach onTimer function to our timer.
+  timerAttachInterrupt(timer, &onTimer, true);
+
+  // Set alarm to call onTimer function every second (value in microseconds).
+  // Repeat the alarm (third parameter)
+  timerAlarmWrite(timer, 1000000, true);
+
+  // Start an alarm
+  timerAlarmEnable(timer);
+}
+
+void loop() {
+  // If button is pressed
+  if (digitalRead(BTN_STOP_ALARM) == LOW) {
+    // If timer is still running
+    if (timer) {
+      // Stop and free timer
+      timerEnd(timer);
+      timer = NULL;
+    }
+  }
+}