Import importnb to this project

[importnb](https://github.com/deathbeds/importnb) is a newer and better version of this package, that does the same things - but is more maintained.

I spoke to @tonyfast and there is interest in making this happen. It will help adoption, and keep this project moving forward.

Things to do:
- [ ] Import contents of that git repo to here, preserving all histories
- [ ] Add maintainers of importnb to this repo
- [ ] Add maintainers of importnb to pypi
- [ ] Make an announcement