Conditions to be published

[brenes]

Right now the menus in this engine are static and they have no reason to not be published if they have their draft attribute to false.

But there are some scenarios where publishing one menu item depends on external information (e.g. a menu item attahet to a product category may be not shown if the category has no available products).

It would be great to have a mechanism to implement this conditionals, although there are cache considerations that must be taken into account: What about the whole leaves_with_draft? scope to avoid queries? What the Rails fragment cache? Should we implement some expiring mechanism?