learn to disable STARTTLS

Some servers might have this feature broken.

Github-ref: https://github.com/OfflineIMAP/offlineimap/issues/207
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/repository/IMAP.py

@@ -261,6 +261,9 @@ class IMAPRepository(BaseRepository):
     def getsslversion(self):
         return self.getconf('ssl_version', None)
 
+    def getstarttls(self):
+        return self.getconfboolean('starttls', True)
+
     def get_ssl_fingerprint(self):
         """Return array of possible certificate fingerprints.