Add a task for uploading nginx site config and a generator for configuring template before uploadinging

add a task ```cap ningx:config``` to upload nginx site config
add a generator ```rails g capistrano:nginx_puma:config``` for customizing template before uploading 
update readme
https://github.com/seuros/capistrano-puma/blob/master/lib/capistrano/tasks/puma.cap#L129-L146