Allow per-field overriding of default_operator

So I can do something like this:
```python
class AlbumFilterSet(FilterSet):
    class Meta:
        model = Album
        query = session.query(Album)
        operators = (Equal, Greater, Less)
        column_overrides = {
            'name': {'default_operator': Like}
        }
```

Author: cript0nauta

filteralchemy/filterset.py

@@ -77,10 +77,12 @@ def get_model_filters(mcs, klass):
                 opts.converter.field_for(opts.model, prop.key)
             )
             operators = overrides.get('operators') or opts.operators
+            default_operator = (overrides.get('default_operator') or
+                                opts.default_operator)
             for operator in operators:
                 operator_name = (
                     operator.label
-                    if operator != opts.default_operator
+                    if operator != default_operator
                     else None
                 )
                 name = underscore_formatter(prop.key, operator_name)