Add support for xdg specification config file (#32)

config/locations.go

@@ -4,11 +4,18 @@ import (
 	"os/user"
 	"path/filepath"
 	"runtime"
+
+	"github.com/adrg/xdg"
 )
 
 func GetLocations() (res []string) {
 	res = append(res, "./cli.json")
 
+	xdgPath, err := xdg.ConfigFile(filepath.Join("gotify", "cli.json"))
+	if err == nil {
+		res = append(res, xdgPath)
+	}
+
 	if usr, err := user.Current(); err == nil {
 		res = append(res, filepath.Join(usr.HomeDir, ".gotify", "cli.json"))
 	}