Feature request: endpoint IP and port config

[fenichelar]

Use case:

ES is running in a VPC without public access. SSH LocalForward is used to access ES. The LocalForward rule must be configured to listen on 443 and /etc/hosts must be configured to make the endpoint resolve to 127.0.0.1.

Proposal:

Add endpoint IP and port config options (endpoint-ip and endpoint-port) so that the IP and port can be specified. The proxy would then send the requests to this port and IP but still sign the requests with the correct endpoint URI. This would allow the LocalForward rule to use a port number that does not require root privileges and would prevent the need for hardcoding the IP in /etc/hosts.

Thoughts? I can take a stab at implementing and submit a PR.