Add support for dotplots

There was a [blog post](http://motioninsocial.com/tufte/) showing how to do many Tufte-style plots in R. The author included `ggMarginal` but noted that it cannot be used to make dotplots eg


I suppose it could be a nice extra feature to support those kind of plots as well, even though they're very very similar to histograms. I looked into it very briefly and unfortunately the `ggplot2::geom_dotplot()` doesn't seem to work because when there are many observations the dots go off the screen. There might be a way to somehow dynamically decide how big to make the dots. There can also be other solutions (perhaps useful resource: http://lenkiefer.com/2016/04/06/dot-plots-and-distributions)

If anybody wants to tackle this, feel free to make a PR!