Apply additional arguments to :cmd after defaults.

This patch gives a clean way of adding additional arguments at the end of the custom command (before the spec files are specified), useful for example if it was necessary to override one of the defaults.

The reason for this patch is to provide a simple hack to enable compatibility with zeus-parallel_tests (see sevos/zeus-parallel_tests#26) 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.) Obviously this isn't very clean because of the assumption that none of the default options have quotes in, but it seems like an acceptable temporary hack that's useful in it's own regard?
Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.