Expose printDirective function to enable schema sharding

[tomgasson]

GraphQL schemas can end up quite large. Sharding the schema into multiple files is supported by clients like relay to allow you to split a large schema into multiple smaller files. This can reduce merge conflicts, and make things easier on the file system, git's operations and your IDE's syntax highlighting.

This PR exports the printDirective function, which will allow you to write your own custom implementation of printSchema for your own sharding mechanism. printType is already exported

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[tomgasson]

Any chance of this being merged in?

[IvanGoncharov]

@tomgasson Merged, thanks for the ping!
If you want this change in v16, please, backport it to https://github.com/graphql/graphql-js/tree/16.x.x