add color functions

[brunoti]

I'm thinking of:

import sketch/color

// Something like this with contructors
type Color {
  Named(String)
  Hex(String)
  Rgb(Int, Int, Int)
  Rgba(Int, Int, Int, Float)
  Hsl(Int, Float, Float)
}

sketch.class([
  sketch.background(
  	color.named("red")
		|> color.alpha(0.5)
		|> color.darken(0.2)
		|> color.to_hsl_string()
	)
])

sketch.class([
  sketch.background(
  	color.rgba(r: 255, g: 255, b: 255, alpha: 0.1)
		|> color.alpha(0.5)
		|> color.to_rgba_string() // rgba(255, 255, 255, 0.5)
	)
])

sketch.class([
  sketch.background(
  	color.rgba(r: 255, g: 0, b: 0, alpha: 1)
		|> color.to_hex_string() // "#FF0000
	)
])

	color.hex("FFF")
		|> color.alpha(0.5)
		|> color.to_hex_string() |> // rgba(255, 255, 255, 0.5)
	)

But let's discuss the API