Add Mandelbrot in color

[certik]

The grayscale Mandelbrot now works (#11), the next step is a color Mandelbrot.

That is probably very easy now, we just need to add a different "callback" from JavaScript to show a color image. For example:

    interface
        subroutine show_img_color(n, m, A) bind(c)
        integer, intent(in) :: n, m
        integer, intent(in) :: A(3,n,m)
        end subroutine
    end interface

Where the first index is the color (three channels R,G,B, each a value between 0..255).

Let's keep options for both grayscale and color, both is helpful to have.