cp -r doesn't match linux when destination doesn't exist

In linux, `new_dir` and `existing_dir` are now copies (`new_dir` didn't exist before):

```
$ mkdir existing_dir
$ touch existing_dir/foo
$ cp -rf existing_dir new_dir
```

With shelljs, `new_dir/existing_dir` and `existing_dir` are copies.  I would expect the linux behavior.  

```
cp('-rf', 'existing_dir', 'new_dir')
```
