Add NoApp() (#33)

app.go

@@ -393,3 +393,10 @@ func handleShutdownErr(a *App, ac AppContext, err error) error {
 	}
 	return err
 }
+
+// NoApp returns a nil app.
+// It can be used when you call a function that accepts a *App, but there's no lu app in scope.
+// f(lu.NoApp()) reads more clearly than f(nil).
+func NoApp() *App {
+	return nil
+}

app_test.go

@@ -370,3 +370,7 @@ func TestWaitFor(t *testing.T) {
 		})
 	}
 }
+
+func TestNoApp(t *testing.T) {
+	require.Nil(t, lu.NoApp())
+}