Returns the fully-resolved data config (base defaults merged
with sparse overrides). If var_name is provided, returns only that
variable's config entry.
config.get(var_name = NULL, options_file_name = NULL, options_dir = NULL)[list] The fully-resolved data config (or a single entry).
[character, optional] A specific variable name to
retrieve. If NULL (default), returns the entire config.
[character, optional] The name of the options
file. If NULL (default), the user will be prompted interactively.
[character, optional] The directory containing options
files. If NULL (default), the default directory is used.