Allow multiple string arguments for exec()

I've found that I end up doing a lot of messy string concatenation with `exec()` commands. It would be nice to allow multiple string arguments that would be joined together with a space, such as:

``` js
exec('npm', 'run-script', 'test');   // run "npm run-script test"
```

This could easily be backwards compatible with the current function since only the first argument is currently a string.
