Add fpm_hook recipe directive.

This feature allows recipe to change fpm attribute.

Package::Dir sets several fixed fpm attributes as default. But some attributes are missing accessor method to change the value from the recipe. The fpm_hook exposes the internal object to the recipe and let user to modify it.

Example:

```
class Recipe < FPM::Cookery::Recipe
  platform :redhat do
    fpm_hook do |fpm|
      fpm.attributes[:rpm_user] = 'httpd'
    end
  end

  platform :debian do
    fpm_hook do |fpm|
      fpm.attributes[:deb_user] = 'apache'
    end
  end
end
```