Added initial support for RGB LED mood / status light(s)

Added initial basic support for using RGB LED(s) as "mood" / status
indicators (eg. initial bed/extruder heating, build finished etc).
Added new M code, M420, with parameters R, V, and B to control red,
green and blue ('V' is used for green as 'G' cannot be used). Values
range from 0 to 255.
An additional parameter, I, can be used as a simple method of
controlling overall intensity/brightness. Values range from 0.0 to 1.0.
Default pins are PWM pins 5, 6, and 7.