Implement specifiedBy directive merged into graphql-js

[Ambro17]

As per issue #649 and the merged pr the reference implementation now includes a new directive.
I understand that this lib is intended as a 1 to 1 port to graphql-js so perhaps we should consider including it.
If not possible, we should document the divergence

[Cito]

This has already been included in 9228af4 and will be available soon with the next release.

[Ambro17]

Thank you!