Update to JupyterLab 1.0.0

[lresende]

Update the xkcd extension to conform to the current APIs in JupyterLab 1.0.0.

Following the repository pattern, this should become a branch with the same version of the JupyterLab release version, thus this should be merged to a 1.0.0 branch.

[saulshanabrook]

@lresende OK if I merge this now?

[lresende]

@saulshanabrook I have removed the wip, but note that following the repo pattern this should end up in a 1.0.0 branch as there is no master... or we could create a master branch... now that jlab is 1.0.0

[parente]

For past versions of JupyterLab, the branches here had a fixed number of commits referenced by the https://jupyterlab.readthedocs.io/en/stable/developer/extension_tutorial.html tutorial. For example:

• https://github.com/jupyterlab/jupyterlab_xkcd/commits/0.35
• https://github.com/jupyterlab/jupyterlab_xkcd/commits/0.34

I see the tutorial has been rewritten with astronomy pictures instead of xkcd comics, so maybe it doesn't matter anymore.

[jasongrout]

I reset master to this PR, and made master the default branch.

[jasongrout]

Thanks for updating this!