Allow to configure enumerability of property

/cc @kamsi

At this point all defined properties, are set as enumerable. There's no way to tweak that.

There are however use cases, when we may want non-enumerable property on an object. Good example is map of same type values, where we want to add some custom property, that should not be treated as part of map collection, and should not be iterable with forEach.

There's a specific use case for that in eRegistrations app, where we have map of sections, and we need a property that will calculate overall status of the sections. At this point we put it aside on an object as sectionsStatus, while more natural would be to have it as sections.status.