added initial basic support for RGB LED mood / status lights

Adds support for the use of RGB LED(s) for use as "mood" lighting or as
a status indicator (eg. initial bed/extruder heating, end of build etc).
Adds a new M code, M420 for control of the LEDs via g-code. Available
parameters are R, V, and B for setting RGB colour ('V' is used for green
as 'G' cannot be used). Values range from 0 to 255. An additional
parameter, I, can be used as an easy method of setting overall
intensity.
By default PWM pins 5, 6 and 7 are used for red, green and blue.
local config change