Add message/channel/guild acknowledging (#1239)

* add acking

* 👀

* Update RESTMethods.js

* Update TextBasedChannel.js

* Update RESTMethods.js

* Update Guild.js

* Update TextBasedChannel.js

* Update Message.js

* super shitty names

* Update GroupDMChannel.js

* Update DMChannel.js

* Update TextChannel.js

src/structures/Message.js

@@ -493,6 +493,15 @@ class Message {
     return this.channel.send(content, Object.assign(options, { reply: this.member || this.author }));
   }
 
+  /**
+   * Marks the message as read
+   * <warn>This is only available when using a user account.</warn>
+   * @returns {Promise<Message>}
+   */
+  acknowledge() {
+    return this.client.rest.methods.ackMessage(this);
+  }
+
   /**
    * Fetches the webhook used to create this message.
    * @returns {Promise<?Webhook>}