Enable custom plugins

[lucperkins]

I'm a huge fan of this library, especially the MDX support 🎉, but I'd like to be able to provide my own extensions (such as super-fancy code blocks with all the bells and whistles and support for fun things like Mermaid diagrams). As far as I can tell, there isn't currently a way to do that. If I'm wrong and there is a way to do that, please let me know and I'll close this 😄 But if this isn't currently possible, I'd like to know if you consider that a non-goal for the project. If it is a non-goal, I'm happy to close this and explore other options. But if you're open to that, I'm happy to help.