Automatically create the cache folder used for external files 

Setting the cache folder (`dj.config['cache'] = '/tmp/dj-cache'`) to a folder that doesn't exist throws a FileNotFoundError when fetching. It would be nice if it created it automatically
Here's the error
```
FileNotFoundError: [Errno 2] No such file or directory: '/tmp/dj-cache/su7VnvVwCN901A7maY3lX4es-_digGrC3mFQtcSXdHEbl3d.saving'
```