add colorwheel generator

[johnnychen94]

using ImageCore
using OffsetArrays

function colorwheel(sz::Dims{2}, v=1)
    # H, S
    canvas = OffsetArrays.centered(fill(ARGB(0, 0, 0, 0), sz))

    for I in CartesianIndices(canvas)
        x, y = I.I ./ (size(canvas) .÷ 2)
        r = sqrt(x*x+y*y)
        if r < 1
            h = atand(x, y) + 90
            canvas[I] = RGB(HSV(h, r, v))
        end
    end
    return canvas
end
colorwheel((1024, 1024))

This generates the widely used color wheel https://commons.wikimedia.org/wiki/File:Color_circle_(RGB).svg

I use this for visualization test purposes. I'm wondering if this should be put here or TestImages.

[kimikage]

One main concern is that Colors.jl does not have the ability to display images. We need ImageShow.jl etc.

Another concern is that user preferences may be diverse.
For example, whether to set 0° to the right or top, clockwise or counterclockwise, what background color, how many margins, etc.
I don't think it is necessary to address all of them, but it is better to be able to extend the wheel with kwargs.

So, how about adding a ColorWheel type that only holds settings, and rendering the instance when display-ed or show-ed, as we do in the Colors.jl documentation?

https://github.com/JuliaGraphics/Colors.jl/blob/f0a508bce74a3c6a15bb01a5ab52bc9b9ba81f73/docs/crosssectionalcharts.jl