Ability for plugins to customize how text is displayed

I'm not sure how best to go about this, but there's definitely demand for custom display of LLM output, including this excellent looking library for rendering streaming Markdown: https://github.com/day50-dev/Streamdown

It's been a very early feature request!

• #12

I don't want to bake this stuff into LLM core but I'd love to enable plugins to do custom stuff like this.