Plots.jl: Allow list of colors to be passed to hline function

[djsegal]

Right now this only produces red lines (i.e. no green ones):

scatter(rand(100))
hline!([0.5,0.25,0.75], color=[:red, :green, :green])

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This can be seen here:

[mkborregaard]

In Plots, columns are series. So to add several series with different attributes use a row vector.

hline!([0.5 0.25 0.75], color=[:red :green :green])

[djsegal]

thanks!