Provide option to listen on specific TCP address

[jochenchrist]

The proxy listens on every TCP address with the given port.

For security reasons (e. g. when running as a Sidecar in Kubernetes), I want to restrict to listen on 127.0.0.1:8080 only.

Please provide an option to bind the http server on a specific TCP address, e. g. like this:

./aws-signing-proxy -listen 127.0.0.1:8080

[jochenchrist]

Just provided a Pull Request: #33