Augmenter le support de Markdown IT

[ffoodd]

• Attribut loading="lazy" pour les images : https://mdit-plugins.github.io/img-lazyload.html#usage
• Support de <abbr> : https://mdit-plugins.github.io/abbr.html
• Support de <ins> : https://mdit-plugins.github.io/ins.html#usage
• Support de <mark> : https://mdit-plugins.github.io/mark.html

Sur d’autres projets (a11y.css, chaarts, sseeeedd) :

• Démo : https://mdit-plugins.github.io/demo.html
• Snippet : https://mdit-plugins.github.io/snippet.html