Allow custom sorting of mailboxes for mbnames

mutt-sidebar and, probably, other MUA show mailboxes in the order
they are listed in the file written by mbnames.  Therefore, to allow
customization of the order with which mailboxes are listed, introduce
the new 'sort_keyfunc' directive in the [mbnames] section.

'sort_keyfunc' must be a function that will be called once for each
mailbox.  It must accept the only argument -- a dict with 2 items,
'accountname' and 'foldername', and should return an object that
will be used as the sorting key for each mailbox.

Default key function returns (d['accountname'], d['foldername']),
thus sorting by account name and then by the folder name.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>