markdown: support for emoji shortcodes #216

Most code forges' Markdown rendering backend support emoji shortcodes to lessen the pain of looking up at Emojipedia (or local equivalents like KDE Emoji Selector or hitting Super + : on Windows)) and copy-pasting the thing. When rendered, they should be converted into Unicode equivalents or use Twemoji SVGs (with their Unicode equivs as alt text) for compatibility between systems.