Enable alpha for Geom.rect

[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

• enables alpha for Geom.rect (and Geom.rectbin)
• fixes Alpha in SVGJS #1353

Example

x1, y1, w1 = 0.5:10, rand(10), 0.09.+0.4*rand(10)
D = DataFrame(x=x1, y=rand(x1, 10), y1=y1, x2=x1.+w1, y2=y1.+w1, c=0:9)
p1 = plot(D, xmin=:x, ymin=:y1, xmax=:x2, ymax=:y2, color=[colorant"green"], alpha=1:10,
    Geom.rect, Scale.alpha_discrete)
p2 = plot(D, xmin=:x, ymin=:y1, xmax=:x2, ymax=:y2, color=:c, alpha=[0.7],
    Geom.rect, Guide.ylabel(nothing))
p3 = plot(D, x=:x, y=:y, color=:c, alpha=[0.5], Geom.rectbin, Scale.color_discrete)
hstack(p1, p2, p3)

[codecov-io]

Codecov Report

Merging #1439 into master will decrease coverage by 0.84%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1439      +/-   ##
==========================================
- Coverage   90.26%   89.42%   -0.85%     
==========================================
  Files          39       39              
  Lines        4140     4369     +229     
==========================================
+ Hits         3737     3907     +170     
- Misses        403      462      +59     

Impacted Files	Coverage Δ
src/geom/rectbin.jl	80.00% <66.66%> (-7.50%)	⬇️
src/poetry.jl	90.74% <0.00%> (-5.26%)	⬇️
src/theme.jl	65.07% <0.00%> (-4.42%)	⬇️
src/geom/label.jl	96.26% <0.00%> (-2.74%)	⬇️
src/geom/beeswarm.jl	90.00% <0.00%> (-2.65%)	⬇️
src/aesthetics.jl	77.77% <0.00%> (-2.06%)	⬇️
src/misc.jl	65.06% <0.00%> (-2.06%)	⬇️
src/Gadfly.jl	76.00% <0.00%> (-1.94%)	⬇️
src/color_misc.jl	60.60% <0.00%> (-1.47%)	⬇️
src/guide/keys.jl	81.81% <0.00%> (-1.24%)	⬇️
... and 18 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 40c708b...e1d6762. Read the comment docs.