Hooks for pre and post generation messages and check

Cookiecutter supports hooks (ref), three scripts that allow the developer to show some messages, validate the user input or perform post-generation operations.

This branch will only introduce the concept of hooks and print messages in the three stages available but this feature can be easily extended to support automatic installation of packages, download of ad-hoc .gitignore files or automatic Gitlab repositories setup. With some work, it should also be possible to implement an automatic sync with the cluster that requires little to no input from the use.