Add support for rx only or tx only operation

[jcwren]

A nice feature to add would be supporting single pin operation, where a controller can be configured to only receive or only transmit without tying up an IO pin for the unused mode.

I realize this can be worked around by specifying an unused IO pin for the mode not being used, but that does assume that not every available IO pin is being used. My particular application has two controllers, and one just needs to periodically send a change of state byte to the other.