add the database_dir option to the config.

I want to store the core data file to the application support directory.
Because when I store it to the document, my app was rejected before.
I need an option to change the directory.

In cdq.yml

```cdq.yml
database_dir: NSApplicationSupportDirectory
```
or 
```cdq.yml
database_dir: NSDocumentDirectory
```

default is NSDocumentDirectory.