Add utility function function for generating fully-featured schema from SDL

With SDL [officially](https://github.com/graphql/graphql-spec/pull/90) part of the specification, would it be worthwhile to provide a way to build a build a fully-featured schema from a Document AST?

Currently, schemas built using `buildASTSchema`/`buildSchema` lack several features that are otherwise available if constructed programmatically:

* Specifying `resolve` functions for fields
* Specifying `resolveType` functions for unions and interfaces and `isTypeOf` functions for types
* Specifying enum value values
* Adding custom scalars

Workarounds exist for some of these limitations (functions can be passed through the root object for root type fields, abstract types can be handled by explicitly providing a `__typename` property, etc.). In practice, though, if I'm starting a new project and want to use SDL to define my schema, I'm forced to use `graphql-tools` or another third-party package. This was completely understandable prior to the 2018 version of the spec, but now it may be appropriate for this functionality to be part of the core library.