support for draft-6 examples metadata

[proppy]

https://tools.ietf.org/html/draft-wright-json-schema-validation-01#section-7.4 added support for examples metadata.

It would be nice if GenSON provided a way to include the leaf scalar values passed to add_object as examples of the corresponding properties.

[wolverdude]

This pending version (1.2) will open up the schema builder for extension, so this should be easy to implement as an add-on, and it could be activated through a seed schema.

I'm interested in implementing some more common keywords in the next version after this. I'm not sure this is common enough to make the cut, but I'll consider it. Please make the case for why it should be included.