add `geom_beeswarm()`

[Zaf4]

I saw you were adding new type of plots. Swarm plot is very useful and it is similar to the violinplot but each point is shown in a way overlap is prevented.

Here is the ggbeeswarm for reference.

[alshan]

Looks interesting, thank you. Note however that some of these charts can be done using geom_dotplot() or geom-ydotplot().