Create a geom for hdr_boxplot

[mitchelloharawild]

Things to do:

• [ ] Identify the things the user should be able to control (width, level, outliers?)
• [ ] Create setup_data() to prepare data for plotting (this is after the stat has been computed!)
• [ ] Create the draw_group(), I've kept the GeomBoxplot$draw_group() here for tweaking.
• [ ] Which aesthetics do we require for this geom? geom_boxplot has ymin and ymax, how does that translate to multiple levels?