add alpha aesthetic to Geom.line

[bjarthur]

fixes #1589

julia> using Gadfly, DataFrames

julia> test = DataFrame(
           x=1:100,
           y=rand(100),
           z = repeat(["a", "b"], outer = 50));

julia> plot(test,
            x=:x,
            y=:y,
            color = :z,
            Geom.line,
            alpha=[0.1])

julia> plot(test,
            x=:x,
            y=:y,
            alpha = :z,
            Geom.line,
            Theme(alphas=[1.0,0.5]))

[codecov-commenter]

Codecov Report

Merging #1603 (e405027) into master (89ca326) will decrease coverage by 0.57%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1603      +/-   ##
==========================================
- Coverage   89.15%   88.57%   -0.58%     
==========================================
  Files          39       39              
  Lines        4942     4954      +12     
==========================================
- Hits         4406     4388      -18     
- Misses        536      566      +30     

Impacted Files	Coverage Δ
src/geom/line.jl	100.00% <100.00%> (ø)
src/scale.jl	86.57% <0.00%> (-4.84%)	⬇️
src/geom/rectbin.jl	80.00% <0.00%> (-2.23%)	⬇️
src/guide.jl	90.13% <0.00%> (-1.05%)	⬇️
src/coord.jl	89.07% <0.00%> (-0.85%)	⬇️
src/mapping.jl	85.24% <0.00%> (-0.82%)	⬇️
src/geom/subplot.jl	93.92% <0.00%> (-0.56%)	⬇️
src/aesthetics.jl	80.64% <0.00%> (-0.53%)	⬇️
src/Gadfly.jl	76.75% <0.00%> (-0.38%)	⬇️
src/statistics.jl	94.26% <0.00%> (-0.28%)	⬇️

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[Mattriks]

I'm surprised that this PR works (GiovineItalia/Compose.jl#386 (comment)), but nice that it does. I will try to think of some examples that it might fail for (due to Compose), but I think this PR is ok as is (we can fix any issues in Compose later).

[bjarthur]

i just added a test. SVG, PDF, and PNG output all work for me. @Mattriks let me know if you find a case if fails.