A list of options.
rcmd_process_options() creates a set of options to initialize a new
object from the rcmd_process class. Its arguments must be named, the
names are used as option names. The options correspond to (some of)
the arguments of the rcmd() function. At least the cmd option must
be specified, to select the R CMD subcommand to run. Typically
cmdargs is specified as well, to supply more arguments to R CMD.