add markdown support
Summary:
Adds support for Markdown documents
BUG: 360603
Test Plan: Open a Markdown (.md) document in Okular
Reviewers: Okular, aacid
Reviewed By: Okular, aacid
Subscribers: michaelweghorn, aacid
Tags: Okular
Differential Revision: https://phabricator.kde.org/D7382