[Task] Permit other default values for the config file

What should be done

Instead of always loading the lenra.yml file as config file, handle many file extensions.

The files will be loaded in the next order and stops, when one exists:

• lenra.yml
• lenra.yaml
• lenra.json

Technical recommandation

Check if the file exists before loading it.

When loading the lenra.json be careful because I'm not sure that serde_yaml handle it perfectly.