Extend voicemail to allow IMAP folders to be specified per mailbox.

Previously only possible per context, new option called imapfolder.

(closes issue #14298)
Reported by: jablko
Patches: 
      patch-200906202 uploaded by jablko (license 675)



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@232700 65c4cc65-6c06-0410-ace0-fbb531ad65f3