Slight modification to handle SD_MMC connections

[HankLloydRight]

Hi,

Great lib, thank you!

I'm using SD_MMC instead of SD/SPI, and in order to get this lib to work, I had to change two lines:

Line 10: #include <SD_MMC.h>
Line 158: _file = SD_MMC.open(_filename, _mode);

And then the repository works perfectly with SD_MMC connections.

[stevemarple]

If the Arduino IDE allowed programmers to externally define macros it would be possible for IniFile to include the appropriate header files and use the correct open function. Despite asking, and submitting a pull-request to support it (arduino/arduino-builder#29), the IDE developers repeatedly refuse to include such a feature, which makes adding this kind of support difficult.