Switch README etc from iris to penguins?

[eddelbuettel]

The four examples in the README work quite well with one added pp <- palmerpenguins::penguins. Here for example is the fourth as I just put it together for a slide:

[grantmcdermott]

Thanks @eddelbuettel. I thought about it a while back, but would prefer to stick with datasets in the base package for the quickstart example. Maybe once/if r-devel/r-svn#154 is merged....

Plot looks good, though! One minor suggestion would be call par(las = 1, lab = c(10,10,7)) beforehand to get better coverage across the axes. I hope to roll this kind of aesthetic automation into a tinytheme() function---no relation---at some point.