add name re-defination and sub-navigators supports

* If I don't like name `'nav'`, simply pass `name` to `Navigation`.
* Add sub-navigators support. just like this:

```python
bar1 = nav.Bar('bar1', [
    nav.Item('item1', 'endpoint1'),
    nav.Item('item2', 'endpoint2'),
])
bar2 = nav.Bar('bar2', [
    nav.Item('item3', 'endpoint3', sub_nav_bar=bar1),
    nav.Item('item4', 'endpoint4'),
])
```

then in endpoint1 or endpoint2, item3 will be active.