Render output by default, without ipywidgets

[zmbc]

First of all, I just want to say that I think this project is an incredibly good idea. I think it fills one of the biggest gaps in the Jupyter ecosystem, which encourages bad coding practices (copy-and-pasting code).

I'm excited to adopt this, but I wish it would automatically render all output from subnotebooks in the parent notebook, in such a way that that output could render even in a static viewer like GitHub. Even better if there were a way to do so inside a collapsible section (like <details>) but I don't think the Jupyter format supports this.

Thoughts on this @davidbrochart? If you think it makes sense, at least as an option, and is feasible, I'd be happy to help implement.

[zmbc]

Hi @davidbrochart any reactions? If I opened a PR for this, would you accept it?