When installing from CRAN, record repository URL and name

[hadley]

e.g. something like this:

options(repos = c("FOO" = "https://cran.rstudio.com"))
install.packages("ggplot2")

packageDescription("ggplot2")
#> ...
#> RepositoryUrl: https://cran.rstudio.com
#> RepositoryName: FOO
#> ...