Support configuring software SPI pins

[navravi]

The Leonardo and Mega use different hardware SPI pins which make them incompatible with the SD library when using the Datalogger shield.

[per1234]

Both the Leonardo and Mega break out the SPI pins to the ICSP header:

Modern well designed shields will make the SPI bus connections there rather than on the board-specific pins. When that is not done, it is more that the shield is not compatible with the board than that the SD library is not compatible with the shield, since the library uses the correct pins on these boards.

The only way for the library to support the use case of plugging the incompatible shield directly to the board would be to bit bang the SPI protocol on user-configured pins. So I suppose we can consider this a request for "software SPI" support, such as what the SdFat library has already:

https://github.com/greiman/SdFat/blob/master/examples/SoftwareSpi/SoftwareSpi.ino

Related: #6