* Add subscriptions support on GraphiQL
Addresses #501
BREAKING CHANGE: `juniper::http::graphiql::graphiql_source` now requires
a second parameter
BREAKING CHANGE: `juniper_hyper::graphiql` now requires
a second parameter
BREAKING CHANGE: `juniper_iron::GraphiQLHandler::new` now requires
a second parameter
BREAKING CHANGE: `juniper_rocket::graphiql_source` now requires
a second parameter
BREAKING CHANGE: `juniper_warp::graphiql_filter` now requires
a second parameter
* Add test where graphiql subscriptions endpoint is not None