Shift to purrr rather than apply for explaining working with lists (and other list stuff)

[gilliganondata]

http://www.dartistics.com/learn-to-love-lists.html

Possibly keep some nod to the apply() functions, but get a bit more pure about using purrr.

Also, consider making the Big Three the Big Four and promting purrr into that list (pun intended) at http://www.dartistics.com/tidyverse.html.

Consider removing the listviewer/jsonedit() content at http://www.dartistics.com/learn-to-love-lists.html now that RStudio has a better list viewer. Or... at least narrow the explanation of when/where jsonedit() comes in handy.