Enable adding a custom field to all nodes.

[wolverdude]

In order to add a custom field to all schema nodes, a user would have to subclass every schema strategy and add code to it for that new field, and then create a new SchemaBuilder class with all those SchemaStrategy subclasses. This is an unreasonable amount of work for such a simple change.

GenSON should have a relatively simple, documented path for this use-case.

See Issue #65 for more context.