XDG_CONFIG_HOME environment variable is ignored on macOS.

Steps to reproduce:

1. set XDG_CONFIG_HOME to e.g. $HOME/.config
2. create configuration file at $XDG_CONFIG_HOME/senpai/senpai.scfg
3. launch senpai

Expected results:

senpai will read configuration file at $HOME/.config/senpai/senpai.scfg.

Actual results:

The configuration file at "/Users/pwr/Library/Application Support/senpai/senpai.scfg" was not found.
Configuration assistant: senpai will create a configuration file for you.