Add ability to inherit custom fields from RailsAdmin::Config::Fields::Association

I'm writing new custom field for globalize3 integration and I noticed that you get `undefined method [] for nil:NilClass` error due to empty `@properties` variable that `association` method returns in `label` method:
``` ruby
register_instance_option :label do
  (@label ||= {})[::I18n.locale] ||= abstract_model.model.human_attribute_name association[:name]
end
```
`field.properties` instance variable is lost when rails_admin calls `configure` method inside edit block:
``` ruby
edit do
  configure :translations, :globalize3_tabs
end
```
`field` method is called inside `configure` method, which removes this field from `_fields` array (because its type hasn't been found in the default stack), and creates it again as my custom type.

In the process, it gets `properties` for this field from an `abstract_model.properties`, but it's `nil`, because it's not a abstract_model's property, but it's an association.