update cat_aes_var!

[Mattriks]

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

This PR:

• updates Gadfly's cat_aes_var! methods
• fixes Julia 1.0 incompatibility in cat_aes_var!() #1228

cat_aes_var! is called by Gadfly.concat, which forms the plot_aes (later used in rendering guides). In Gadfly, all aesthetics are 1d vectors, not arrays (except for z).
With this PR, z arrays fallback on the general cat_aes_var! function.

Examples (from #1228):

p1 = plot( layer(x=[5], y=[1.0], shape=[Shape.star1]),
    layer(x=1:10, y=rand(10), Geom.point, shape=[Shape.circle], alpha=[0.1]),
    Theme(point_size=4pt, discrete_highlight_color=identity, key_position=:none) )

Zi = [(x,y) for x in -5:0.5:5, y in -5:0.5:5]
fn1(x) = x[1]*exp(-hypot(x[1]/3, x[2]/3)^2); fn2(x) = cos(hypot(x[1], x[2])/0.5)+1.0
i, ls = -5:0.5:5, 0.2:0.2:0.8
p2 = plot(Coord.cartesian(fixed=true),
    layer(z=fn2.(Zi), x=i, y=i, Geom.contour(levels=[1.0]), Theme(line_style=[:dash])),
    layer(z=fn1.(Zi), x=i, y=i, Stat.contour(levels=[-ls; ls]), Geom.polygon(fill=true)),
    Scale.color_continuous(minvalue=-1, maxvalue=1.0), Theme(lowlight_color=identity) 
)
hstack(p1, p2)

[codecov-io]

Codecov Report

Merging #1333 into master will increase coverage by 0.1%.
The diff coverage is 60%.

@@            Coverage Diff            @@
##           master    #1333     +/-   ##
=========================================
+ Coverage   90.02%   90.12%   +0.1%     
=========================================
  Files          36       36             
  Lines        3941     3941             
=========================================
+ Hits         3548     3552      +4     
+ Misses        393      389      -4

Impacted Files	Coverage Δ
src/aesthetics.jl	81.03% <60%> (+0.86%)	⬆️
src/statistics.jl	96.94% <0%> (+0.1%)	⬆️
src/geom/polygon.jl	100% <0%> (+5.12%)	⬆️

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[bjarthur]

LGTM. i'd squash the commits when merging. thanks!

[Mattriks]

Done.