Add RecipesBase plotting recipe

[nalimilan]

This fixes plotting with Plots.jl, preserving the custom ordering of levels.

Fixes #256.

@daschw @mkborregaard Does this sound correct? I had to adjust the recipe you wrote at JuliaPlots/Plots.jl#2519 (comment) to support missing values. Since the recipe only has access to CategoricalValue entries and not the the parent CategoricalArray, I couldn't find a way to detect whether the array has a Union{<: CategoricalValue, Missing} eltype, so I always include missing in the set of possible values. But AFAICT plot automatically skips them when they are not present so I guess it's OK?

Also I've tried to write basic tests without relying on heavy checks of the produced graphs, let me know if there's a better way.

[daschw]

Hi, sorry for the late response! This looks good IMO. I tested on all backends if there are any issues with your missing hack, but it works fine. Unfortunately, I don't know of a better way to test recipes without producing graphs, but I guess your tests check everything the recipe does, so this should be fine.

[nalimilan]

Thanks!