Support ALB

https://aws.amazon.com/about-aws/whats-new/2018/11/alb-can-now-invoke-lambda-functions-to-serve-https-requests/

Starting an issue to work out the how the API would work. I think doing something like:

```python
from chalice import Chalice

app = Chalice(app_name="helloworld", listener='alb')
```

omitting listener will default to 'apigateway'

Because things like authorizers just aren't compatible across listeners, it makes sense to make listener part of the app construction, as opposed to something that is decided at deploy time. I'm sure there is more I'm not thinking of.

Start by reviewing the proposed Chalice app construction with listener='alb' and the existing default listener='apigateway' behavior. Work out how listener-specific features such as authorizers should be represented and define what ALB support and the listener API must accomplish.

Written by the indexing model from the issue text.