graphiqlOptions should support being called as a function that is passed the request

[NeoPhi]

We have a need to dynamically determine the graphiqlOptions that should be used on a per request. This would mirror the semantics of graphqlOptions supporting a function.

This approach could also be used to address #399 by examining the incoming Host header to dynamically generate the subscriptionsEndpoint and/or endpointURL.

[helfer]

@NeoPhi I think that's a great idea. Do you have time to make a PR that would allow both an object and a function as graphiqlOptions? 🙂