Add a way to configure the GraphiQL subscription endpoint

The current GraphiQL interface served by the handler sends GraphQL subscriptions over HTTP, which is not expected to work. [This article](https://dev-blog.apollodata.com/how-to-use-subscriptions-in-graphiql-1d6ab8dbd74b) describes how GraphiQL can be set up with a custom fetcher that does subscriptions over a GraphQL WebSocket.

It would be great if the handler would support this to work with GraphQL servers that implement WS subscriptions.