Merge the `graphql-relay-subcription` into `graphql-relay-js`?

[noghartt]

Could we merge the features of taion/graphql-relay-subscription into this package? In my opinion, would be good to unify the features into this package.

[saihaj]

We plan to move this package to graphql-js in next major release. I will check it out and I suppose we can move that too 🤔 cc @IvanGoncharov