An app can have several a list of root media

An app can have several FileFields that are stored at different directories. So django-orphaned should contemplate this usecase supporting that the configuration of the app have a list of paths instead a string.
```
ORPHANED_APPS_MEDIABASE_DIRS = { 
    'fooapp': {
        'root': IMAGE_ROOT,  # only one path (setting is a string)
        'exclude': ('.gitignore')
    },  
    'barapp': {
        'root': (BAR_TEMPLATE_ROOT, BAR_DATA_ROOT),  # several path (setting is a list)
    },  
}
```