Jun2018 spec: Handle type extensions

[jlouis]

Rather than doing this work as part of #182, and in #161 we created this task for handling extensions of type schemas. It is a really nice idea, and we should definitely support it. But it is a separate issue and to keep focus in #182 I'm trying to avoid it on a first pass.

[jlouis]

Probable implementation path:

• Support something like graphql:extend_schema(File) or if we have multi-tenancy: graphql:extend_schema(Schema, File). This will extend the current schema with the new definitions inside File and "monkey patch" the existing schema.
• Typical use case: support a local schema definition for internal use only, but inherit everything from the public schema so it is a strict superset.