0.2.5 release

fix #19, improve support multi device (#21)

- add ESP32 HSPI / VSPI support (Kudo's to Alex Uta, PR #22)
  - add **performance_0.2.4.md** for ESP32
- add example **MCP23S17_two_SELECT.ino** (#19)
- add example **MCP23S17_two_ADDRESS.ino** (#19)
- add **void enableHardwareAddress()** (#19)
- add **void disableHardwareAddress()** (#19)
- add keywords.txt (#20)
- update readme.md
- minor edits