Avoid using the protection to a route of better_errors, fixes #62.

This recipe will inject `set :protect_from_csrf` into `app/app.rb` after `Padrino::Application`.
So, `app/app.rb` will be something like this.

```ruby
module Foobar
  class App < Padrino::Application
    set :protect_from_csrf, except: %r{/__better_errors/\d+/\w+\z}
    register Padrino::Rendering
    register Padrino::Mailer
    ...
  end
end
```