This adds a basic freeform ability to mark Plugins as "child_only" and optionally, who can be its Parents 

This is a really simply patch that should mirror the allow_children / child_classes functionality.

Intent:  

1. the ability to mark a Plugin that can ONLY be nested and
2. If provided, exactly which Plugins this Plugin can be nested under.

Obvious use cases for this are related plugins that depend on eachother in terms of markup, and or a Plugin that has an explicit one to many relationship with some other class of Plugin.