Adds SSL support

Adds HTTPS configuration to nginx-proxy.

I decided to keep the certificate-files outside the container, so they're exposed by adding a volume from /path/to/nginx/certificates:/etc/nginx/certificates

It's assumed that all virtualhosts uses the same certificate (since that's how we're using it)

I couldn't figure out how to make it optional in the template.

I was inspired by @paimpozhil comment on pull request #10