Matching arg field (#1)

Check if the first field of an argument branch matches the parent struct field.

kong_test.go

@@ -98,10 +98,21 @@ func TestArgSlice(t *testing.T) {
 	require.Equal(t, true, cli.Flag)
 }
 
-func TestUnsupportedfieldErrors(t *testing.T) {
+func TestUnsupportedFieldErrors(t *testing.T) {
 	var cli struct {
 		Keys map[string]string
 	}
 	_, err := New("", "", &cli)
 	require.Error(t, err)
 }
+
+func TestMatchingArgField(t *testing.T) {
+	var cli struct {
+		ID struct {
+			NotID int `arg:""`
+		} `arg:""`
+	}
+
+	_, err := New("", "", &cli)
+	require.Error(t, err)
+}