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.

options_test.go

@@ -10,8 +10,8 @@ func TestOptions(t *testing.T) {
 	var cli struct{}
 	p, err := New(&cli, Name("name"), Description("description"), Writers(nil, nil), ExitFunction(nil))
 	require.NoError(t, err)
-	require.Equal(t, "name", p.Model.Name)
-	require.Equal(t, "description", p.Model.Help)
+	require.Equal(t, "name", p.Name)
+	require.Equal(t, "description", p.Help)
 	require.Nil(t, p.Stdout)
 	require.Nil(t, p.Stderr)
 	require.Nil(t, p.Exit)