Adding support for Collapsable headings

[rpytel1]

The current version of nbstripout does not support metadata changes if you use popular adds on: collapsable heading from nbextensions.

Current behaviour

When using collapsable heading extension when a section is closed or open information is saved in Jupyter Notebook.

Desired behaviour

We want to limit all the changes to notebook to useful changes, so information, whether the user is using the extension, is irrelevant.

Fix

As this extension is essentially adding two new fields to the metadata field (either "hidden":True or none and "heading_collapsed:True" or again None) it is only needed to add this fields to ingore line.

Sidenote: Current version from master of repository is not working so for my own purposes I removed last 5 commits to revert to Pypi version.

This PR is adding a new commit on top of master.

[kynan]

Thanks for your contribution! Another argument for switching to an allowlist #86 :)