Takes vectors of colours and partial_colour_ramps and produces
colours by interpolating between each from colour and the target colour
the specified amount (where amount and from are the corresponding
fields of the ramp).
For example, to add support for the fill_ramp aesthetic to a geometry,
this line could be used inside the draw_group() or draw_panel() method
of a geom:
data$fill = ramp_colours(data$fill, data$fill_ramp)