add begin/end transaction helpers (needed for really odd devices like SHARP memory display which has inverted CS)

Adafruit_SPIDevice.cpp

@@ -172,6 +172,25 @@ uint8_t Adafruit_SPIDevice::transfer(uint8_t send) {
   return data;
 }
 
+/*!
+ *    @brief  Manually begin a transaction (calls beginTransaction if hardware
+ * SPI)
+ */
+void Adafruit_SPIDevice::beginTransaction(void) {
+  if (_spi) {
+    _spi->beginTransaction(*_spiSetting);
+  }
+}
+
+/*!
+ *    @brief  Manually end a transaction (calls endTransaction if hardware SPI)
+ */
+void Adafruit_SPIDevice::endTransaction(void) {
+  if (_spi) {
+    _spi->endTransaction();
+  }
+}
+
 /*!
  *    @brief  Write a buffer or two to the SPI device.
  *    @param  buffer Pointer to buffer of data to write

Adafruit_SPIDevice.h

@@ -51,6 +51,8 @@ public:
 
   uint8_t transfer(uint8_t send);
   void transfer(uint8_t *buffer, size_t len);
+  void beginTransaction(void);
+  void endTransaction(void);
 
 private:
   SPIClass *_spi;