Closes #217. Use XDG-compatible config directory

This change is largely experimental and it's entirely possible it could
wipe out your existing configuration. Be warned.

Old config path was: ~/.wtf/
New config path is:  ~/.config/wtf/

If an existing config directory already exists, this change attempts to
copy it to the new location.

Note that if your config file contains paths to files in the old config
directory, they won't work. You'll need to change them by hand.

logger/log.go

@@ -89,7 +89,7 @@ func logFilePath() string {
 		return ""
 	}
 
-	return filepath.Join(dir, ".wtf", "log.txt")
+	return filepath.Join(dir, ".config", "wtf", "log.txt")
 }
 
 func (widget *Widget) tailFile() []string {