Added possibility to use other SPI device

[Saur0o0n]

This is small change, that does not affect currently working scripts, that allow to use another SPI device - except default one.
ESP32 has three of them - if you already use default SPI for something else - there was no way to use any other in this library.
With this change, you can do nothing - and everything work's as before, or you can declare new SPI device and pass it to the object.

SPIClass *ESP32_SPI = new SPIClass(HSPI);
/* start MAX31855 */
myMAX31855.begin(ESP32_SPI);