Update Stat.qq

[Mattriks]

• I've updated the documentation to reflect these changes
• I've added an entry to NEWS.md
• 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 color aesthetic for Stat.qq
• fixes Being able to specify color for Stat.qq #1429

Example

using DataFrames, Distributions
 y = randn(100).+5
ds = fit.([Normal, LogNormal], [y]) 
df = [DataFrame(y=randn(100), g=g) for g in ["Sample1", "Sample2"]]

theme = Theme(discrete_highlight_color=c->nothing, alphas=[0.5], 
    point_size=2pt, key_position=:inside)
yeqx(x=-3:3) = layer(x=x, Geom.abline(color="gray80"))
xylabs = [Guide.xlabel("Theoretical q"), Guide.ylabel("Sample q")]

p1 = plot(x=Normal(), y=randn(100), Stat.qq, yeqx, xylabs..., theme,
    Guide.title("1 sample, 1 Distribution"))
p2 = plot(vcat(df...), x=Normal(), y=:y, Stat.qq, color=:g, yeqx, 
    xylabs..., theme, Guide.title("2 samples, 1 Distribution"))
p3 = plot(x=ds, y=y, color=["Normal", "LogNormal"], Stat.qq, yeqx(3:8), 
    xylabs..., theme, Guide.title("1 sample, 2 Distributions"))
p = hstack(p1, p2, p3)