Highlight markdown in issue/comment editor

Description

Markdown may look easy for us coders, but getting markdown right for non-tech folks is less appealing, and the back and forth between Write and Preview tabs of the issue/comment editor when something isn't parsed right is sometimes frustrating, especially because showing one hides the other.

Proposal

Highlighting markdown as you type helps in nailing markdown on the first pass and hints at what's wrong (and what's right!) straight in the issue/comment editor.