Export graphqlKoa

Being able to access graphqlKoa is useful for koa-based frameworks that don't expose the app object. This was previously exported in an earlier version of apollo-server and would be useful to export in the 2.x release as well.

Here's an example of how we're currently consuming this: https://github.com/fusionjs/fusion-plugin-apollo-server/blob/90e95ef1480659073c7e4e042b75ea6205cc068f/src/server.js#L37

packages/apollo-server-koa/src/index.ts

@@ -27,3 +27,7 @@ export {
   registerServer,
   ServerRegistration,
 } from './ApolloServer';
+
+export {
+  graphqlKoa,
+} from './koaApollo';