Allow the user to pass a site template file

This is something I started using while I was going through the Deis cookbooks: https://github.com/opdemand/deis-cookbook/blob/master/definitions/nginx_site.rb

This addition allows users to pass a template and variables to create an nginx site. I use it in a few of my own cookbooks like this:

```
nginx_site "munin" do
    template "site.erb"
    variables ({
        :listen => node.munin.port,
        :root => node.munin.www
    )}
    action :enable
end
```