Add tree view in manage.py show_urls

[kleutzinger]

It would be nice to be able to format the urls in the tree view as something like this.

python manage.py show_urls --format tree

$ python manage.py show_urls --format tree
://:80
├── admin
│   ├── <app_label>
│   ├── auth
│   │   ├── group
│   │   │   ├── <path:object_id>
│   │   │   │   ├── change
│   │   │   │   ├── delete
│   │   │   │   └── history
│   │   │   ├── add
│   │   │   └── autocomplete
│   │   └── user
│   │       ├── <id>
│   │       │   └── password
│   │       ├── <path:object_id>
│   │       │   ├── change
│   │       │   ├── delete
│   │       │   └── history
│   │       ├── add
│   │       └── autocomplete
...

(note: i used https://github.com/DriftSec/urltree to generate the above)