Fictionbook: Add support for code, superscript, & subscript tags
Summary: BUG: 387426
Test Plan: Open a book with the code, superscript, & subscript tags. Verify that the code is visible in a mono-space font, & superscripts & subscripts are formatted appropriately.
Reviewers: Okular, aacid
Reviewed By: Okular, aacid
Subscribers: aacid, cfeck, ngraham
Tags: Okular
Differential Revision: https://phabricator.kde.org/D10566