Generalize fills

Generalize fills so we can use different ones everywhere.

• string - A color, just like now.
• (elem, ...) => fill - Function that returns a fill.
• ClockFill object - An object that performs the fill.

There should be helper functions:

• gradient(type, coords, stops) - return a function that draws a gradient.

There should also be color utilities:

• color.hsl(h, s, l) - convert from hsl to rgb.
• color.lighten(color, amount) - lighten a color.
• color.darken(color, amount) - darken a color.
• color.mix([w, color], ...) - weighted mix of colors.

And probably others.

Look in to the possibility of specifying fill colors as rgba(r, g, b, a) in the fill parameter.