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