Add additional arguments to custom rspec command.

A simple hack to enable compatibility with rspec-parallel_tests via
these (or similar) options:

    cmd: 'zeus parallel_tests -o "',
    cmd_additional_args: '"'

(parallel_tests requires all rspec's options to be passed via its
`-o` option.)

However this patch also gives a cleaner way of adding additional
arguments at the end of the argslist (e.g. if it was necessary to
override one of the defaults) so has merit on its own.