Support for stream module

I wanted to float an idea: support for streaming connections. I put together a quick proof of concept on the stream branch of this fork. Run docker-compose -f docker-compose-separate-containers.yml up -d --build to see connections proxied to both the whoami service and a mongo service. You can grab a mongo admin UI and connect to it using mongo.local:27017.

So, what do people think? Is there a valuable use case for this?

Downsides:

• You can't have different VIRTUAL_HOSTS listening on the same port, since there's no hostname resolution like with HTTP.

Work left:

• Get it working when docker-gen and nginx are running in the same container
• Block http access over the streaming port
• Finish stream.tmpl
• Tests