Maildir repository: add config keys for IMAP keywords

This commit assembles a dictionary mapping user-specified IMAP keywords
to Maildir lower-case flags, similar to Dovecot's format
http://wiki2.dovecot.org/MailboxFormat/Maildir

Configuration example:

[Repository Local]
type = Maildir
localfolders = ~/Maildir/
customflag_a = $label1
customflag_b = $Forwarded
customflag_c = Junk

Signed-off-by: Igor Almeida <igor.contato@gmail.com>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/repository/Base.py

@@ -133,6 +133,9 @@ class BaseRepository(CustomConfig.ConfigHelperMixin, object):
     def getsep(self):
         raise NotImplementedError
 
+    def getkeywordmap(self):
+        raise NotImplementedError
+
     def should_sync_folder(self, fname):
         """Should this folder be synced?"""