MCP23018 compatibility

[uppfinnarjohnny]

I've tried out the library with the MCP23018 , and it (not surprisingly) seems to work just fine. I don't know if it's necessary to include in the readme, but now it at least is written somewhere. :-)

[johntalton]

the *18 device has slightly different iocon registers and will not behave as expected (specifically when using interrupts)

[caternuson]

Yep. So it probably mostly works. But could run into issues when hitting those differences.