Add query_label to give each filter a unique label

flask_admin/model/filters.py

@@ -72,6 +72,14 @@ class BaseFilter(object):
         """
         raise NotImplemented()
 
+    def query_label(self):
+        """
+            Return a string that can be used in a url to identify this filter.
+            
+            For example u'username_equals'
+        """
+        return '{name}_{operation}'.format(name=self.name, operation=self.operation()).lower().replace(' ', '_')
+
     def __unicode__(self):
         return self.name