utf8foldernames: support --delete-folder with UTF-8 folder name

Github-ref: https://github.com/OfflineIMAP/offlineimap/issues/505
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

docs/offlineimap.txt

@@ -160,9 +160,10 @@ blinkenlights, machineui.
 --delete-folder::
   Delete a folder on the remote repository.
 +
-Only one account must be specified/configured for this feature to work. The
-folder name must be provided in IMAP encoding with the remote separators (likely
-'/'). E.g.: "Remote/folder/name".
+Only one account must be specified/configured for this feature to work or you
+must provide one account with -a. The folder name must be provided with the
+remote separators (likely '/') in UTF-8 if utf8foldernames is enabled or in IMAP
+otherwise. E.g.: "Remote/folder/name".
 
 
 --migrate-fmd5-using-nametrans::

offlineimap/repository/IMAP.py

@@ -524,6 +524,8 @@ class IMAPRepository(BaseRepository):
     def deletefolder(self, foldername):
         """Delete a folder on the IMAP server."""
 
+        if self.account.utf_8_support:
+            foldername = imaputil.utf8_IMAP(foldername)
         imapobj = self.imapserver.acquireconnection()
         try:
             result = imapobj.delete(foldername)