Expect rare words to be in the extended dictionary

codespell_lib/_codespell.py

diff --git a/codespell_lib/_codespell.py b/codespell_lib/_codespell.py
index ca45d6f..9ad62b4 100755
--- a/codespell_lib/_codespell.py
+++ b/codespell_lib/_codespell.py
@@ -40,7 +40,7 @@
 # The aspell tests here aren't the ideal state, but the None's are realistic
 # for obscure words
     ('clear', 'for unambiguous errors', '', False, None),
-    ('rare', 'for rare but valid words', '_rare', None, None),
+    ('rare', 'for rare but valid words', '_rare', True, None),
     ('informal', 'for informal words', '_informal', True, True),
     ('code', 'for words common to code and/or mathematics', '_code', None, None),  # noqa: E501
     ('names', 'for valid proper names that might be typos', '_names', None, None),  # noqa: E501