Finalize configuration system

[MythicManiac]

Currently the configuration system is rather empty compared to what it could be.

Ideally most (if not all) options that can be configured via the thunderstore.tm could also be provided via environment variables or CLI arguments. Understandably some complex datatypes (such as build mappings) might be difficult to support nicely via environment variables or CLI arguments, so some thought is required in how this is done in practice.

Moreover, the user-scoped configuration file which should hold cross-project configurations (such as auth tokens) is currently entirely implemented.

This issue exist as a placeholder for more detailed specifications, mostly as a reminder that the configuration system still needs work.