Add a gitfilter to clean the output and metadata from an ipynb

[basnijholt]

Regarding #1617 and #2507.

What about this?

You activate the filter with:

git config filter.nbstrip_jq.clean './nbstrip_jq.sh'
git config filter.nbstrip_jq.smudge cat
git config filter.nbstrip_jq.required true

[jlstevens]

Thanks @basnijholt for the suggestion!

It is good to have a concrete proposal like this but I think there is more discussion to be had:

1. We would prefer to have a solution that doesn't require jq
2. Can the filter only apply to specific directories?
3. Very occasionally we might want to commit notebooks with some output. How would you handle that if such a situation arises?
4. We probably need an approach that also helps contributors who aren't regular enough developers that they would want to set up the git filters. For instance, a common, user-friendly command...
5. My ideal git filter would be one which applies when you stage a change, before you make the commit. This would give me the option to see the diff e.g if I work on a new machine where I haven't got the git filters configured.

At any rate, I think once we've thought it all through, git filters should be documented for the developers who want to use them.

[basnijholt]

This is another solution using nbconvert although it doesn't do all of the things @jlstevens mentioned above.

[philippjfr]

Will close and maybe use your nbconvert solution.