Add client.cacheUser method (#548)

* Add client.cacheUser method

* Fixed ESLint issues

* Added existence check first

src/client/Client.js

@@ -135,6 +135,16 @@ class Client extends EventEmitter {
     return this.rest.methods.loginToken(email);
   }
 
+  /**
+   * Forces a user to be cached.
+   * @param {String} id The ID of the user to cache
+   * @return {Promise<User>}
+   */
+  cacheUser(id) {
+    if (this.users.has(id)) return Promise.resolve(this.users.get(id));
+    return this.rest.methods.getUser(id);
+  }
+
   /**
    * Returns a Collection, mapping Guild ID to Voice Connections.
    * @readonly