Add missing `__init__.py` file in `plugins/`

[jorgepiloto]

🐞 Problem

This sub-pacakge is not intended to be imported by users and thus, no __init__.py file lives in it.

However, from the point of view of Python package, any sub-package requires to have an __init__.py file to be defined as "sub-package". Also, for documentation purposes, the automated doc-tools need a "healthy" package layout to be able to generate documentation.

[jorgepiloto]

Closing this issue in favor of #28.