header-ids: avoid conflicting ids (fixes #661)

Author: pmjdebruijn

lib/markdown2.py

@@ -1560,7 +1560,7 @@ def header_id_from_text(self,
 
         self._count_from_header_id[header_id] += 1
         if 0 == len(header_id) or self._count_from_header_id[header_id] > 1:
-            header_id += '-%s' % self._count_from_header_id[header_id]
+            header_id += ':%s' % self._count_from_header_id[header_id]
 
         return header_id