Consider adding script to spell check jupyter notebooks

[sjspielman]

I was also thinking about whether we want a second script for checking jupyter notebooks.

File an issue for it! One idea I had was to export the notebook as markdown using nbconvert and then spell check that? It will be a bit harder to go through the output for fixes as line numbers will be wrong, but should still be doable.

Originally posted by @jashapiro in #2 (comment)

[jashapiro]

Followup idea here: A lighter version may be to just convert the markdown chunks, which can be done with jq:

<file.ipynb jq -r '.cells[] | select(.cell_type == "markdown") | .source | join("")'  > file.md