Make the Logger configurable via a .cfg file

[akien-mga]

Right now everything can be configured, but would have to be done in the _init, _enter_tree or _ready of the main scene.

The singleton should have a mechanism to save and load its configuration (default parameters, defined modules and their parameters).

This could likely be done by appending the project's engine.cfg with a logger section, or maybe using an own logger.cfg file. I'm not sure how saving the settings should be handled though, as these settings would typically be overridden at runtime, and then saving them would imply that they're saved in user://. So that doesn't prevent the developer from having to define everything manually at least once somewhere.

Thoughts?