Updated package rendering so that opts are available in template

Some modifications to rendering of files, also for better support of the `defaults` option of my other PR when one wants to use these defaults in a template.

The ERB rendering doesn't use the local binding anymore, but now merges `opts` and given options (second argument of both `render` and `template`) and use that as context.

Example:
This
```ruby
package :nginx do
  file '/etc/nginx/nginx.conf', :contents => render('nginx', :worker_processes => opts[:worker_processes] || 2)
end
```

with the template:
```erb
user www-data;
worker_processes <%= @worker_processes %>;
pid /run/nginx.pid;

events {
	worker_connections 768;
	# multi_accept on;
}
...
```

NB. This causes that local variables won't be passed on to the template anymore.