Add CNAME option

When a repo have a CNAME file, it will use the URL from it to map the server_name on ngnix.

@progrium let me know your thoughts about this. The implementation is really simple, and IMO gives a lot, so we can add a custom domain to an app without using the VHOST.

I have about 5 apps running in the same host machine, every container has a completely different domain, So thats how I make it work. Just need to add a file called `CNAME` to the root of my repo, and ngnix will use that url. (see https://github.com/arthurnn/octostalker/commit/138ab8bbe94263e17c4b07408f35b54e592f8203 for example)