Gapminder

* `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()`.

* Added the `tbl_df` class to the `gapminder` data frame, which is advantageous for users of the `dplyr` package.
* Changed (corrected?) the `population` variable from numeric to integer. Affected India (all years) and China (1952).
* Moved imputation of 1952 China data earlier in the data cleaning process, which added a row to `inst/gapminder-unfiltered.tsv`.
* Added the `gapminder_unfiltered` data frame. It's the data frame `gapminder` came from, but is less heavily filtered (previously available only in `inst/gapminder-unfiltered.tsv`).
* Added tab-delimited files for the country and continent colors, `inst/continent-colors.tsv` and `inst/country-colors.tsv`.
* Added description of the "international dollars" in which GDP per capita is reported (thanks aammd, 5).

* Initial CRAN release