Versioning

Similar to https://github.com/plasmabio/tljh-repo2docker/issues/67, having tags and releases would make it easier to use the plugin with different versions of TLJH and JupyterHub.

We should consider making a `v1.0.0` from the latest commit on master, to allow for other changes like https://github.com/plasmabio/plasma/pull/217 and https://github.com/plasmabio/plasma/pull/224 to land afterwards.