Fixes valueless arguments; e.g. --colors

At the moment trying to use the following arguments results in an error: `--invert`, `--colors`, `--no-colors`, `--growl`, `--debug`, `--bail`, `--async-only`, `--sort`, `--debug-brk`, `--check-leaks`, `--inline-diffs`.

This fixes that by not appending the value to the arguments list unless it is a string or a number. So all of the following work:

```javascript
{
  invert: true,
  colors: null,
  debug: undefined
}
```

Thoughts ?