configitems: register the 'notify.config' config

hgext/notify.py

@@ -145,6 +145,7 b' from mercurial import ('
     error,
     mail,
     patch,
+    registrar,
     util,
 )
 
@@ -154,6 +155,13 b' from mercurial import ('
 # leave the attribute unspecified.
 testedwith = 'ships-with-hg-core'
 
+configtable = {}
+configitem = registrar.configitem(configtable)
+
+configitem('notify', 'config',
+    default=None,
+)
+
 # template for single changeset can include email headers.
 single_template = '''
 Subject: changeset in {webroot}: {desc|firstline|strip}