Added more ggplot2 examples in extras/ #mozsprint

[stephenturner]

Added more material and examples to the ggplot2 lesson in extras.

• dealing with overplotting
  • alpha
  • geom_hex
  • stat_density2d
• adding new layers to plots
• geom_jitter
• geom_boxplot
• geom_violin
• geom_smooth
• plotting statistics derived from data with stat_summary
• combining aesthetics, mappings, scales, other options to create non-traditional graphics, with notes of caution about creating senseless visualizations.

[jdblischak]

@stephenturner, what are your thoughts on this PR in light of #759 (summarized in Greg's blog post)? I think it would make sense to develop this into its own lesson repository.

[stephenturner]

Agreed, or at least in a separate R repo. I believe Karthik has some good material somewhere as well that I've seen referenced here.

[jdblischak]

OK. Great. We can revisit the discussion of where to place these materials once we see the state of the repos post-breakup of bc.