jennybc/gapminder: v0.3.0

country_codes is a new data frame that contains ISO 3166-1 country codes (#16 @jrebane). Import tibble::tibble(), so tibble printing is used out of the box. Moved delimited files for practicing data import into inst/extdata/. Therefore, these are now accessible after installation at, e.g., system.file("extdata", "gapminder.tsv", package = "gapminder"). Clarify that this package is not maintained as a definitive data source, rather it's for making code examples and teaching. Stability is now very important. Improved citability, e.g. added the "concept" DOI that links to a list of all version to DESCRIPTION and README. README also shows the use of citation().