Feature: Support multiple strands with shared clock

[Gadgetoid]

Clocking out multiple strands of APA102 pixels should be possible by using a separate data pin for each strand, combined with a single shared clock pin. It's not possible to use this configuration with hardware SPI, but software GPIO mode should readily support this.

It would be nice if this were as transparent as possible- perhaps by setting up multiple APA102 classes with the same clock pin and using a custom class that hooks them and clocks out the data using a combined method.

Strands may or may not need to be the same length- it may be possible to pad the buffers with 0x00 or 0xFF to avoid needing to match strand length.