Support GraphQL extends syntax

[cjhowedev]

GraphQL supports an extend keyword when used with Apollo. This adds fields to a type. I think all that we would need to do is add the extend keyword to the list of keywords. It looks like this:

extend type Query {
  myField: String!
}

I don't know if this is out of scope because it's specific to Apollo afaik.

[cjhowedev]

I just updated and this works great. Thanks a lot!