Add option for prepending timestamp to generated file

[brookback]

I love this tool! 😍 It was exactly what I looked for. Thanks so much!

It would be cool though with an option (or even as a default) to have a timestamp added to the top of the generated types .ts file, in order to quickly diff the version at a glance.

Something like:

import { GraphQLResolveInfo } from 'graphql';

/**
 * This file is auto-generated by graphql-schema-typescript
 * Please note that any changes in this file may be overwritten
 *
 * Generated at 2018-07-20T23:01:01
 */

/* tslint:disable */ 
/*******************************
 *                             
 *          TYPE DEFS          
 *                             
 *******************************/

Let me know if I can provide a PR for this.