Automatic OpenAPI 404 response

The idea is, if an endpoint's URL rule contains a variable, then add a 404 response for it:

```python
@app.get('/pets/<id>')
def get_pet(id):
    pass
```

So user don't need to set a 404 response manually:

```python
@app.get('/pets/<id>')
@doc(responses=[404])
def get_pet(id):
    pass
```

Also, add a config to control this behavior:

```
AUTO_404_RESPONSE: bool = True
```