More robust config machinery

[lagru]

Many of the requested enhancement in this repository would likely profit of a more robust configuration machinery. The current system is very basic with almost no validation of the config file. Stepping that up a bit would be useful. Maybe with an approach like https://github.com/scientific-python/docstub/blob/80a7ce455d8256ca772632b5a0c27b63ee649d0f/src/docstub/_config.py or something else that makes it more robust.