feat: allow user specific configs

[alandtse]

User configs will be saved in CommunityShadersUSER.json. This will be loaded first and then fallback to CommunityShaders.json.

We should probably add logic that will invalidate a config if its version is too old using the Version key.

closes #66

[alandtse]

Actually, I think forcing a save may be enough to populate all the necessary values on version change.

[doodlum]

I think this only makes sense if it contains changed values. Otherwise users will be unsure what they changed, and we probably need code to retroactively update configs where we have changed the default value.

[alandtse]

Code as written will try to read the existing config. If it can, it will then check version and then save the new format. So it should have the same effect.