Use XDG config directory as default config path

- Look for config at $XDG_CONFIG_HOME/converge/config.yaml by default
- --config still overrides to an explicit path
- Falls back to single-repo --repo mode if no config file found
- New defaultConfigPath and configFileExists helpers in Converge module

converge.example.yaml

@@ -1,5 +1,6 @@
 # Example converge configuration file.
-# Usage: converge --config converge.yaml
+# Place at ~/.config/converge/config.yaml to use by default,
+# or pass explicitly with: converge --config converge.yaml
 
 repos:
   - path: /home/jbrechtel/org