Support PCF8574 I2C port expanders

[dromer]

Commonly used for adding additional digital i/o pins to Arduino projects.

Particularly the well known HD44780 to I2C adapters make use of these. Thus I'm especially hoping we could then also simply assign these pins when using the HD44780 library, instead of having to use the onboard pins of the Daisy.

[takumi-ogata]

Thank you for this feature request, dreamer!