Separate validation into a distinct step.

This allows help to be called even when the parse trace is invalid.
Without this, the command-line would have to be valid in order to use
help at all, which defeats the purpose.

help_test.go

@@ -0,0 +1,31 @@
+package kong
+
+import (
+	"bytes"
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestHelp(t *testing.T) {
+	var cli struct {
+		String string `kong:"help='A string flag.'"`
+		Bool   bool   `kong:"help='A bool flag.'"`
+
+		One struct {
+		} `kong:"cmd"`
+	}
+	w := bytes.NewBuffer(nil)
+	exited := false
+	app := mustNew(t, &cli,
+		Name("test-app"),
+		Description("A test app."),
+		Writers(w, w),
+		ExitFunction(func(int) { exited = true }),
+	)
+	_, err := app.Parse([]string{"--help"})
+	require.NoError(t, err)
+	require.True(t, exited)
+	fmt.Println(w.String())
+}