Flag Scale.color_none for deprecation

[Mattriks]

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

This PR

Scale.color_none was not the right design, see #527 (comment). This PR flags Scale.color_none for deprecation, in favour of e.g. plot(..., Geom.contour, color=[colorant"black"]).

• fixes Contour plot with all lines in one color #527

[codecov-commenter]

Codecov Report

Merging #1469 into master will decrease coverage by 0.15%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##           master    #1469      +/-   ##
==========================================
- Coverage   89.32%   89.17%   -0.16%     
==========================================
  Files          39       39              
  Lines        4395     4398       +3     
==========================================
- Hits         3926     3922       -4     
- Misses        469      476       +7     

Impacted Files	Coverage Δ
src/scale.jl	94.84% <0.00%> (-2.75%)	⬇️
src/Gadfly.jl	75.48% <100.00%> (ø)

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