Defining scales with function

[AndreiKingsley]

Add an API for defining scale as a actual function.

Something like that:

scale = continuous {
   x -> Color.rgb(0, 0, x / 100 * 256)
}

scale = categorical { x ->
   when (x) {
      "" -> LineType.SOLID
   }
}

Implementation = direct applying + identity scale (#402), but there's problems with column id, should we save both original and transformed column, etc.