Color grouping for density2d

[Mattriks]

• I've updated the documentation to reflect these changes
• I've added an entry to NEWS.md
• I've run the regression tests
• I've built the docs and confirmed these changes don't cause new errors

This PR

• Enables color grouping for Geom.density2d.
• If color is used, then the contour lines are mapped to linestyle.
• Closes Geom.density2d with color grouping #1475

Example

using RDatasets
geyser, iris = dataset("datasets", "faithful"), dataset("datasets", "iris")
geyser.g = geyser.Eruptions.>3

p1 = plot(geyser, x=:Eruptions, y=:Waiting, color=:g, linestyle=[:solid], 
        Geom.density2d(levels=4), Geom.point, alpha=[0.4])
p2 = plot(iris, x=:SepalLength, y=:SepalWidth, color=:Species,  Geom.density2d(levels=4), Geom.point, alpha=[0.8])
hstack(p1, p2)

[codecov-io]

Codecov Report

Merging #1508 (7b65f26) into master (d78d866) will increase coverage by 0.78%.
The diff coverage is 96.77%.

@@            Coverage Diff             @@
##           master    #1508      +/-   ##
==========================================
+ Coverage   88.94%   89.72%   +0.78%     
==========================================
  Files          39       39              
  Lines        4395     4625     +230     
==========================================
+ Hits         3909     4150     +241     
+ Misses        486      475      -11     

Impacted Files	Coverage Δ
src/geom/line.jl	100.00% <ø> (ø)
src/statistics.jl	96.45% <96.77%> (-0.01%)	⬇️
src/ticks.jl	91.09% <0.00%> (-0.28%)	⬇️
src/shapes.jl	100.00% <0.00%> (ø)
src/geom/point.jl	100.00% <0.00%> (ø)
src/geom/ribbon.jl	100.00% <0.00%> (ø)
src/geom/violin.jl	100.00% <0.00%> (ø)
src/geom/boxplot.jl	100.00% <0.00%> (ø)
src/geom/polygon.jl	100.00% <0.00%> (ø)
... and 26 more

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