location of package on internet. The url
should point to a zip file, a tar file or a
bzipped/gzipped tar file.
name
optional package name, used to provide more
informative messages
subdir
subdirectory within url bundle that
contains the R package.
config
additional configuration argument (e.g.
proxy, authentication) passed on to
GET.
before_install
a function that can modify the
contents of the package source directory prior to
installation. Accepts two parameters: bundle (the full
path to the downloaded package zip file) and pkg_path
(the path where the package source is staged prior to
i