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 installatio