Make r4ds package

[hadley]

To store any datasets, and to make it easier for people to get all the packages they need.

[garrettgman]

Let's remember to add

common <- rcorpora::corpora("words/common")$commonWords
fruit <- rcorpora::corpora("foods/fruits")$fruits
sentences <- readr::read_lines("harvard-sentences.txt")

from the strings chapter to the package.

[hadley]

All the data has now been moved into individual packages.