Jupytext support/running a script after checkout

[jupytext](https://github.com/mwouts/jupytext) allows to track jupyter notebooks as e.g. plain python files and converts and synchronizes changes between the ``*.ipynb`` files and the ``*.py`` files. For this to work with ``nbgitpuller``, we would need to run e.g.

```sh
find -name '*.jupy.py'|xargs jupytext --set-formats py,ipynb
```

after each checkout.

Generally it would be enough to allow ``nbgitpuller`` to run an arbitrary shell script after each checkout (this could probably also be achieved with a git post checkout hook, but these would need to be installed after the initial cloning of the repository or with git template option).