add jupytext config in post-init hook

post-init.sh

 #!/usr/bin/env bash
+
+# Update notebooks in the demo directory from upstream repo
 git submodule foreach git pull
 rm -r demo && cp -r jupytext/demo demo
+
+# Copy jupytext's jupyter config to open md and script files as notebooks by default.
+mkdir -p ${HOME}/.jupyter/labconfig
+cp jupytext/binder/labconfig/* ${HOME}/.jupyter/labconfig
+
+# Create the notebook for our jupytext demo
+jupytext demo/get_started.md --to ipynb --update-metadata '{"jupytext":null}'
+
+# Remove the markdown representation
+# (the demo starts with just the ipynb file)
+rm demo/get_started.md
+
+# Trust our World Population notebook
+jupyter trust demo/World\ population.ipynb