Path: add 'depth' argument to 'dirname' (recursion)

```javascript
dirname('/home/foo/bar/test', 2)
// => /home/foo
dirname('/home/foo/bar/test', 0)
// => /home/foo/bar/test
dirname('/home/foo/bar/test', 1)
// same as the normal dirname call
// => /home/foo/bar
```