Rename Item API

Hi @JohnSundell ,

I'm enjoying the Files API and I'm glad I can help in any way. Even if my contributions are just discussion points that lead to potential pull requests from others.

I have, yet another, discussion point to raise. I hope you don't mind.

### RenameItem

Unless I examine the source, it's not clear here if `newName` is `name only` or `name + extension`.

```
public func rename(to newName: String, keepExtension: Bool = true) throws
```

Further, I currently have a situation where the above API is inconvenient.

At the moment I'm unpacking a .pdf file from a CKAsset instance (CloudKit).

1. After digging through the asset, I find a file named asdkfjkajsdkfha.**owkheorhjdsf**
2. The file is stored in temp dir so I need to also move it.
3. I want the file to have a different name, including a new `.pdf` extension.

Perhaps, we might want to have something like the following?

```
public func rename(to newName: String) throws
```

And maybe we can examine the name to see if it contains a `"."`

Do you think this is a valid issue and do you have any ideas on this?