Update .pot files

[rffontenelle]

From command-line interface:

cd data.table
R -e 'if (!("potools" %in% installed.packages())) install.packages("potools"); potools::po_extract()'

or from R interactive shell:

install.packages("potools") # i notgnore if already installed
potools::po_extract()

[MichaelChirico]

Please note in the PR description your method for updating the .pot files, it will be useful for future readers.

(this can be done after merging)

[rffontenelle]

Updated the PR description with the commands.