Add gradient border

[fun840]

This adds some new options to the script, allowing you to have gradient borders.

The --border-gradient option accepts a list of RGBA colors, which will make up the gradient.

The --rainbow option will create a rainbow gradient (using the built in colorsys module's hsv_to_rgb)
The --rainbow-steps option specifies how many colors the rainbow gradient should use, the default being 20. This can slightly affect performance, but around 20 seems to be the sweet spot where performance is good but both ends of the gradient are roughly the same color.
The --rainbow-saturation option specifies the saturation of the colors in the rainbow gradient, this is nice to get a sort of pastel look.
The --rainbow-brightness option specifies the brightness of the colors in the rainbow gradient, for customization and completeness.

The --absolute-gradient option makes the gradient 'absolute,' so it doesn't move with the current window. This gives the effect that the border is masking the gradient.

[Ari-43]

I have been having some fun messing around with this. Are there any plans for this to get merged eventually?