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