The `SubFolders` option in mbsync controls the folder naming style in
the maildir. There are three different styles:
* Verbatim - <maildirPath>/top/sub/subsub
* Maildir++ - <inboxPath>/.top.sub.subsub (used by Dovecot)
* Legacy - <maildirPath>/top/.sub/.subsub
Previously, the `SubFolders` option was hardcoded to `Verbatim`. This
change allows configuration of the `SubFolders` option.