SPI support

[natevw]

Would be cool if johnny-five had SPI support, it would enable compatibility with libraries like https://github.com/natevw/node-nrf and https://github.com/tessel/sdcard.

I think we've talked about this off and on through the years, and I know Firmata lacks [or lacked?] support for the SPI protocol, but I couldn't find reference to that on this project. So I'm mostly filing this as a placeholder I can cross-reference, and of course it'd be great if this becomes a rally point for an implementation too!

[nebrius]

We were discussing this very thing in gitter the other day IIRC (can't seem to find it at the moment). I think the idea was that someone would throw together a proposal and implement it in one of the non-firmata based platforms first to see how things look and go from there.

[soundanalogous]

I'm beginning work on SPI for Firmata: firmata/protocol#26.

I also have a Galileo rev 2 on order so I can work on defining a SPI interface for J5 that will work both on Linux and Arduino.

[rwaldron]

Moving to: rwaldron/io-plugins#3