Inline variable insertion in markdown

[chrisjsewell]

@agoose77

so at this point I feel we would benefit from some wider input from the community (in particular long-standing contributors to Jupyter) to get a sense of where things should go.

Indeed.
Hopefully to focus feedback, I have also created Proposed schema additions for inline variables by chrisjsewell · Pull Request #230 · jupyter/nbformat · GitHub with the proposed additions of the (optional) execution_count and expressions keys to the markdown cell (if that is the route we want to go)