Remove CategorizedHelp from App and allow subcommands to have categories

Just place all subcommands in categories, the default category will be
"" which will properly format the output (and group commands that have
no category).

Also allow subcommands to have categories.

Lastly, augment the test to check the output.

app_test.go

@@ -943,7 +943,6 @@ func TestApp_Run_Version(t *testing.T) {
 func TestApp_Run_Categories(t *testing.T) {
 	app := NewApp()
 	app.Name = "categories"
-	app.CategorizedHelp = true
 	app.Commands = []Command{
 		Command{
 			Name:     "command1",
@@ -981,6 +980,13 @@ func TestApp_Run_Categories(t *testing.T) {
 	if !reflect.DeepEqual(app.Categories(), expect) {
 		t.Fatalf("expected categories %#v, to equal %#v", app.Categories(), expect)
 	}
+
+	output := buf.String()
+	t.Logf("output: %q\n", buf.Bytes())
+
+	if !strings.Contains(output, "1:\n    command1") {
+		t.Errorf("want buffer to include category %q, did not: \n%q", "1:\n    command1", output)
+	}
 }
 
 func TestApp_Run_DoesNotOverwriteErrorFromBefore(t *testing.T) {