add executable arguments to spawn

A slight modification to inject node arguments into sub-commands. Specifically, I made it to allow the --harmony flag to perpetuate to sub-commands.

I'm not quite sure how to write a test for this as no tests exist to test the function as is. See: [#290](https://github.com/tj/commander.js/issues/290)
How about 
`args = process.execArgv.concat(args)`