Allow the number of nested content items returned by the Content Delivery API to be configurable

[blockingHD]

Lets say there are 3 document types A, B, and C. A has a content picker for B and B a content picker for C. Currently when requesting A from /content/item/{Guid for A}?expand=all only the properties for B are available and not C.

The proposal is to have a config value or a depth query string to allow for the properties of content C to be returned.