Add option to invoke parallel task using 'nice' flag

We seem to get intermittent failures when we run our test suite using parallel_test. I managed to consistently reproduce these failures on a single test in isolation by running `n` instances of `yes >/dev/null &` (where n is the number of cores - 1). The failures stopped when I changed the command above to `nice yes >/dev/null &`. So the "nice" option in parallel_test is really valuable for us.

However we have our own rake task that explicitly invokes the parallel:test task rather than running `parallel_spec` directly from the command line so this PR makes it so you can pass along the nice flag like you can currently do for pattern, count, and option args.