use config file stored in XDG_CONFIG_HOME

[TheJolman]

This can store the user's GitHub token. We already support loading the token with environment variables, but it would be more user friendly to support both. One should have precedence over the other (the env var should prob override the value in the config file if it's present).

For those unfamiliar, XDG_CONFIG_HOME is usually ~/.config, so we could use something like ~/.config/oss-stats/config.toml. I'm suggesting toml here because I like it and it's very common in the Python ecosystem, but you can realistically use any config file format here (json, yaml, etc).