Allow schemaPrinter to be customized / extended.

Some libraries are having to copy the schemaPrinter code to do custom stuff to the output, (`@apollo/federation`, `type-graphql`). These libraries require outputting type and field level directives since this isn't supported in this library. Is there any support to do so, or can the schemPrinter just export the other functions so we don't have to copy every function or, even better, create a SchemaPrinter class so everything can be over-ridden, then `printSchema` options can set the printer for BC:

```javascript

type Options = {
commentDescriptions?: boolean,
printer?: SchemaPrinter
};

const defaultPrinter = new SchemaPrinter();

export function printSchema(schema: GraphQLSchema, options?: Options): string {
const printer = options.printer || defaultPrinter;

return printer.print(
schema,
n => !isSpecifiedDirective(n),
isDefinedType,
options,
);
}

export function printIntrospectionSchema(
schema: GraphQLSchema,
options?: Options,
): string {
const printer = options.printer || defaultPrinter;

return printer.print(
schema,
isSpecifiedDirective,
isIntrospectionType,
options,
);
}
```