Forward SSL arguments

Hello ; I needed SSL on my dev environment. So It's a small commit forward some SSL options to thin.

Here an example of my config.ru :
```ruby
  application = File.expand_path('../sharypic.ru', __FILE__)

  options = { application: application }
  if ENV['THIN_SSL_ENABLED']
    options[:ssl] = true
    options[:ssl_verify] = true
    options[:ssl_cert_file] = ENV['THIN_SSL_CERT_FILE']
    options[:ssl_key_file] = ENV['THIN_SSL_KEY_FILE']
  end

  backend = Heroku::Forward::Backends::Thin.new(options)
  proxy = Heroku::Forward::Proxy::Server.new(backend, { host: '0.0.0.0', port: port })
```