Add heatmap support

[mrkn]

We can support heatmap as UnicodePlots.jl did at JuliaPlots/UnicodePlots.jl#88

[nanobowers]

Hello @mrkn , I think adding this may require a dependency on another Gem such as https://github.com/janlelis/paint to be able to support 24-bit color in the terminal. Is it acceptable to add this dependency to support the heatmap?

[mrkn]

I want to investigate the library and consider what should I do.

[mrkn]

We need to use https://github.com/red-data-tools/red-colors and https://github.com/red-data-tools/red-palette to implement colormap used in heatmap because this library is a backend of https://github.com/red-data-tools/charty.

[nanobowers]

@mrkn I am working on this and have made some progress getting simple plots to display. There is a dependency on #34, as it requires at least 256 color support for this feature.