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