transfer arugments for Plugin.finish from view_func (update_listing e.g.)

Right now if you want set `update_listing` as `True`, you need write about the following:
```python
plugin.finish(items=items, update_listing=True)
plugin._end_of_directory = True
```
This patch allow return `dict` from `view_func` and passes this `dict` to `plugin.finish` in `plugin._dispatch`. Also allows return `Iterable` value, it's passed as `items`.