add end() to de-init i2c peripheral

Adafruit_I2CDevice.cpp

@@ -35,6 +35,12 @@ bool Adafruit_I2CDevice::begin(bool addr_detect) {
   return true;
 }
 
+void Adafruit_I2CDevice::end(void)
+{
+  _wire->end();
+  _begun = false;
+}
+
 /*!
  *    @brief  Scans I2C for the address - note will give a false-positive
  *    if there's no pullups on I2C

Adafruit_I2CDevice.h

@@ -10,6 +10,7 @@ public:
   Adafruit_I2CDevice(uint8_t addr, TwoWire *theWire = &Wire);
   uint8_t address(void);
   bool begin(bool addr_detect = true);
+  void end(void);
   bool detected(void);
 
   bool read(uint8_t *buffer, size_t len, bool stop = true);