Add support for cb7 comic book archives
Test Plan: Open a cb7 file (e.g. Attachment 122002 from Bug 410696) using Okular and see if it works.
Reviewers: Okular, aacid
Subscribers: broulik, pino, okular-devel
Tags: Okular
Differential Revision: https://phabricator.kde.org/D23037