Add IntervalTimer.priority()

teensy3/IntervalTimer.cpp

   *PIT_TCTRL = 0;
   *PIT_LDVAL = newValue;
   *PIT_TCTRL = 3;
+  NVIC_SET_PRIORITY(IRQ_PIT_CH, nvic_priority);
   NVIC_ENABLE_IRQ(IRQ_PIT_CH);
 
 }

teensy3/IntervalTimer.h

     reg PIT_LDVAL;
     reg PIT_TCTRL;
     uint8_t IRQ_PIT_CH;
+    uint8_t nvic_priority;
     ISR myISR;
     bool beginCycles(ISR newISR, uint32_t cycles);
   public:
-    IntervalTimer() { status = TIMER_OFF; }
+    IntervalTimer() { status = TIMER_OFF; nvic_priority = 128; }
     ~IntervalTimer() { end(); }
     bool begin(ISR newISR, unsigned int newPeriod) {
 	if (newPeriod == 0 || newPeriod > MAX_PERIOD) return false;
 	return begin(newISR, (float)newPeriod);
     }
     void end();
+    void priority(uint8_t n) {
+	nvic_priority = n;
+	if (PIT_enabled) NVIC_SET_PRIORITY(IRQ_PIT_CH, n);
+    }
     static ISR PIT_ISR[NUM_PIT];
 };