Add ability to generate random themes

[paracycle]

There should be a way to generate random themes that would be pleasing to the eye. Don't know enough about harmonious colors and color theory to pull it off though. Any thoughts @atesgoral?

[atesgoral]

It wouldn't be too hard to add a "randomize" button to:

1. Randomly pick a base HSV colour (I'd normalize all components to [0-1) for cleaner computation)
2. Randomly pick a colour scheme, or use a fixed scheme
3. Pick the complementary colours based on the scheme - e.g. just offset the initial hue by 1/3, 1/2 etc.
4. Use luminance computation to pick light/dark text (see what we did for the RelayRobin theme customizer) -- this might be the trickiest part to keep the text readable
5. Go from HSV to RGB

Some inspiration:

http://paletton.com/
http://app.coolors.co/
https://color.adobe.com/create/color-wheel/
http://www.perbang.dk/color+scheme/

I can jsfiddle around with this when I have the time...

[atesgoral]

Could use https://github.com/avp/spectra