Allow the documentation of custom filtering

On occasion I need to write custom filters for resources that are hard to generalise into an all-purpose heuristic.

When this happens, I would like to be able to declaratively state the documentation output I want on my Resource class, and then have that passed through to the Swagger JSON endpoint. e.g.: 

```Python
class AwesomeResource(ModelResource):
    class Meta:
            model = Awesome
            queryset = Awesome.objects.all()
            filtering = {
                "awesome_code": ALL,
            }
            custom_filtering = {
                'granularity': {
                    'dataType': 'string',
                    'required': True,
                    'description': 'Granularity of the data you want to retrieve'
                }
            }
```

...where custom_filtering becomes a query parameter in the swagger docs