Feat: support settings reloading

[agoose77]

Currently we cache the md instance on the renderer. This means that when the settings are modified, the existing IRenderer instances never reflect the new changes.

Either we make getMarkdownIt return the same object where the settings are unmodified, or we listen to the settings changed signal. I can't think that we necessarily need a unique MarkdownIt instance per-renderer, so perhaps the former option is best.