Contribute improvements for notebook 4.2 install, conda packaging

[Peter Parente]

I'd like to help out with the packaging and distribution of this extension:

• make the project buildable with setuptools (#7)
• include notebook 4.2 nbextension metadata so that the extension can be easily managed using the improved jupyter nbextension CLI instead of activating the JS from within the server extension
• submit a conda-forge feedstock that auto-installs / activates the extension in the sys-prefix conda env
• see if there's a good way to reasonably configure the notebook paths during extension activation as well
• update the README

If I send these as PRs, do you have cycles to review and merge them?