Add fade instruction

Add an instruction to fade from one color frame to another.

This would be a top-level JSON value called `fade`. Default value is `false`. If `true`, then after each frame is set, it will fade to the next frame.

Fading will be done at 25ms intervals (this needs to be tested to ensure it can work on a Pi Zero with a lot of lights). The fading algorithm will be:

* Determine the difference between the R, G and B values of current color and the next color.
* Divide the frame time by 25ms to work out how many intermediate frames are needed.
* Divide the difference between each color by the number of frames, and adjust the color by this difference

To hold colors between frames, set the same color for 2 frames in a row