Archiving the repo

[vidartf]

As this repository has been merged into the core of jupyterlab, this repository will be archived. The code now lives in https://github.com/jupyterlab/jupyterlab/tree/master/packages/celltags and https://github.com/jupyterlab/jupyterlab/tree/master/packages/celltags-extension

[vidartf]

The current README says "Note: A new version of celltags is currently being developed to be merged into core JupyterLab. This repo is at end of lifecycle, and will only be updated with critical bugfixes.", so I guess the repo should be kept open a little longer.