Make sure to expose the active private variable

Fixes #13

Author: oprypin

mkdocs_section_index/__init__.py

@@ -17,6 +17,8 @@ def __init__(
         Section.__init__(self, title=title, children=children)
         self.is_section = self.is_page = True
 
+    active = Page.active
+
     def __repr__(self):
         return "Section" + Page.__repr__(self)