Add `as_command` decorator to simplify adding of new commands

Convert this:

```python
nbdev_thing = thing.nbdev_thing.__wrapped__ # remove call_parse

@delegates(nbdev_thing)
def thing(**kwargs):
nbdev_thing(**kwargs)
nbdev_thing.rich_help_panel = 'Amazing thing'
nbdev_thing.no_args_is_help=False
```

to this:

```python
@as_command('Amazing thing')
@delegates(nbdev_thing)
def thing(**kwargs):
nbdev_thing(**kwargs)
```