Only use tibbles if tibble installed

[hadley]

This uses a couple of tricks (a delayed binding and an .R file in the data/ directory) to conditionally use tibbles if tibble is installed. I think this gives the best of both worlds — if you're teaching a tidyverse centric course, you'll get tibbles; if you're running in a low dependency environment, you'll get a data frame.

The penguins object will never behave differently in a given session; it will either be a pure data frame or a tibble after it is accessed for the first time. It will behave differently in another session if you later install tibble, but I think this should be a relatively rare scenario.