Box-and-whisker plot and updated violin plot

Hi Eveveryone,

I added support for box-and-whisker plots to the plot package and made some updates to the violin renderers that were added recently by Cameron Moy. I put up a pull request with the changes, and if you have time and are interested, you are welcome to review it and provide feedback.

Both the box-and-whisker and violin plots will be available in Racket 8.5 (i.e. it will skip the next Racket release).

Here is what the two plot renderers look like: