cli: Add app delete to CLI

Signed-off-by: Wang Baoling <chinamerp@gmail.com>
I think this would be better as a `for` loop:

```go
fmt.Print("Are you sure you want to delete the app %q? (yes/no): ")
loop:
for {
	var answer string
	fmt.Scanln(os.Stdin, &answer)
	switch answer {
		case "y", "yes":
			break loop
		case "n", "no":
			return nil
		default:
			fmt.Print("Please type 'yes' or 'no': ")
	}
}

if err := client.DeleteApp(appName); err != nil {
	return err
}

...
```