Sometimes we might want host-side software to control it, or even to just turn it off.

This is necessary for reprogramming a Fadecandy board without access to the physical button.

The firmware is feature-complete now, and this will save folks the trouble of downloading a huge toolchain. Plus I like to keep tabs on the quality of the assembly output :)

This makes the mapping from 16-bit color to LUT entry straightforward, while still fully defining the entire range from 0x0000 through 0xFFFF. This fixes an edge case where colors greater than 0xFF00 would wrap past the end of the R/G LUTs onto the next color channel.