Support more than one controller

[RandomInsano]

Currently, I'm relying on the SPI enable line to bring the controller to attention. There should optionally be some arbitrary pin to use so that more than one controller can be used per bus.

[RandomInsano]

Arbitrary pin done. Need to expand that to take an array of pins.

[RandomInsano]

Oh previous me, you had the right idea. Made it an optional field in 0.2 but I'll have to change the API again.

Interesting other angle: Create a completely separate library that abstracts multiple SPI devices and just use that one.