Add xeus-cling demo

[SylvainCorlay]

• add the xeus-cling C++ kernel and a demo notebook
• make use of the conda package for R instead of the combination of apt-get and CRAN.

[SylvainCorlay]

This is ready to go! I checked that the binder works.

You can try it here: https://mybinder.org/v2/gh/SylvainCorlay/jupyterlab-demo/xeus-cling?urlpath=lab/tree/demo/Lorenz.ipynb

[SylvainCorlay]

Thanks!