Use XDG user directories by default instead of current working directory

[RogueOneEcho]

Currently we use the current working directory for

• Loading config.yml
• Saving files

This approach is fine for docker but not ideal for other implementions

Therefore to unblock #181

We should use the dirs crate to locate the appropriate XDG or appdata dir.

[RogueOneEcho]

Completed in fdb675b