Support for rscript_args = "*"

[HenrikBengtsson]

Add support for specifying "*" in rscript_args, which is then expanded to whatever makeClusterPSOCK() like to inject. This would make it possible to do (related to #8):

cl <- parallelly::makeClusterPSOCK(1L, rscript = file.path(R.home("bin"), "R"), rscript_args = c("--no-echo", "--no-restore", "*", "--args"))

Note how we need to have that --args after the CLI code for launching the worker in order for it to work.