Add support for interface extension

[ckolderup]

graphql-js 0.13.1 added support for extend interface, but when merging schemas using graphql-tools, it seems that any definitions that start with those two words aren't included in the schema extensions and are thus ignored when doing something like merging schemas.

It would be extremely helpful to be able to extend an interface in the process of stitching schemas the same way you can extend a type. I can't produce a Launchpad link since it's still running graphql-js 0.12.x but here is the behavior I would expect vs what I'm getting:

original schema:

interface A {
  helloWorld: String
}

schema extension (with appropriate resolver code defined for object A):

extend interface A {
  goodnightMoon: String
}

EXPECTED: calling mergeSchemas with the schemas (and their corresponding resolvers) would result in:

interface A {
  helloWorld: String
  goodnightMoon: String
}

ACTUAL RESULT:

Error: A.goodnightMoon defined in resolvers, but not in schema