Add support for un-equal segments

[ddanny]

The rainbow colors are equally distributed across the interval. Supporting segments that are not equal would be great.

[anomal]

You can use setSpectrum to distribute colours unequally by repeating the same colour. For example, if you would like a gradient from red to black, but you want more black than red, you can use:

rainbow.setSpectrum('red', 'black', 'black');