support vec markersize

[Moelf]

• support vector markersize
• add test

fix #29

julia> xs = rand(1:4, 500);

julia> ys = randn(500);

julia> beeswarm(xs, ys; color = xs, markersize=xs.*2)

cc. @HelloYanming for spotting lack of this feature in Seaborn

[Moelf]

can I get a review 👀 @jkrumbiegel