Warn about nonsensical config option 'sep' for IMAP repositories

We autodetect the folder separator on IMAP servers and ignore any 'sep'
setting in the repository section for IMAP servers. Detect if there is
such a setting and warn the user about it.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

Changelog.draft.rst

@@ -44,5 +44,7 @@ Changes
   both the local AND remote name which caused unwanted behavior in
   combination with nametrans rules. Clarify in the help text.
 
+* Some better output when using nonsensical configuration settings
+
 Bug Fixes
 ---------

offlineimap/repository/IMAP.py

@@ -33,6 +33,9 @@ class IMAPRepository(BaseRepository):
         self._host = None
         self.imapserver = imapserver.IMAPServer(self)
         self.folders = None
+        if self.getconf('sep', None):
+            self.ui.info("The 'sep' setting is being ignored for IMAP "
+                         "repository '%s' (it's autodetected)" % self)
 
     def startkeepalive(self):
         keepalivetime = self.getkeepalive()