support for extending types

[dnalborczyk]

It would be nice to support the extend keyword for graphql schema type definitions, it currently doesn't seem to work for graphdoc.

example:
http://dev.apollodata.com/tools/graphql-tools/generate-schema.html#extend-types

apparently it's a [yet] undocumented feature:
graphql/graphql-js#166

simple example/test case:

# Query seems to have at least one field defined, not sure if that will be the case in the future
type Query {
  _dummy: Boolean
}

extend type Query {
  getTask: String
}

schema {
  query: Query
}