Load config file on startup

[Piturnah]

Keep user configurations in some kind of config file that's loaded at startup

Probably makes sense to keep this in a consistent configuration directory (e.g. ~/.config on Linux - see dirs or directories)

For now it might make sense to just put it in the project root since that might be easier for testing/development process, but I'm not sure yet.

EDIT: just occurred to me that perhaps this is a bad idea as we don't particularly want the in-use settings file to be under source control

Maybe check ./ first before checking config file? Or, check ./ with --features dev or some other flag? Or on cfg!(dbg_assertions)

Eventually this file will be editable in-game via settings menu.