Layer aesthetic inheritance improvement

[Mattriks]

• I've updated the documentation to reflect these changes
• I've run the regression tests
• I've built the docs and confirmed these changes don't cause new errors

This PR:

• Geoms in layers can inherit aesthetics from the plot, if layer aesthetics are incomplete

Example:

using RDatasets
iris = dataset("datasets", "iris")
plot(iris, x=:SepalLength, y=:PetalLength,
    layer(Geom.smooth(method=:loess), color=["Smooth"]),
    layer(Geom.point, color=["Points"]),
    Scale.color_discrete_manual("black","deepskyblue")
)

[codecov-commenter]

Codecov Report

Merging #1463 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1463   +/-   ##
=======================================
  Coverage   89.40%   89.41%           
=======================================
  Files          39       39           
  Lines        4399     4402    +3     
=======================================
+ Hits         3933     3936    +3     
  Misses        466      466           

Impacted Files	Coverage Δ
src/Gadfly.jl	75.88% <100.00%> (+0.05%)	⬆️
src/geom/rectbin.jl	80.00% <100.00%> (ø)
src/statistics.jl	96.45% <100.00%> (+<0.01%)	⬆️

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