Allow to set custom cache key through options

Author: matteodem

src/cache.js

@@ -7,8 +7,11 @@ function cache(func, options) {
     if (!context.resolverCache) {
       throw new Error('Missing resolverCache property on the Graphql context.');
     }
+    
+    const key = options.key 
+      ? options.key(root, args, context) 
+      : `${hash(func)}:${hash(toSafeObject(root))}:${hash(toSafeObject(args))}`;
 
-    const key = `${hash(func)}:${hash(toSafeObject(root))}:${hash(toSafeObject(args))}`;
     const executeAndCache = () =>
       Promise.resolve(func(root, args, context)).then((value) => {
         context.resolverCache.set(key, value, options);