*heavily based* on
nyan-mode.el by Jacek "TeMPOraL" Zlydach <temporal.pl@gmail.com>

... I mean, this is basically Jacek's code with small modifications:
- no more cat
- no more animations
- no more music
- no more static XPM images for the rainbow and the outerspace
- the rainbow is now customizable using up to six custom colors