GraphQL: Expose custom directives in introspection API

[CarleneCannon-Conner]

The GraphQL spec does not allow introspecting custom directives. This feature was requested here.
In the comments, somebody who works on the .NET implementation mentioned that they have implemented it, and looking at their commit history it seems it's been implemented here. We should consider contributing a similar feature to the JavaScript implementation.
Asked about it here

Initial exploratory work by @skywickenden

Originally added by @simoneb

References
graphql/graphql-spec#300
graphql/graphql-js#2942
graphql/graphql-js#3047