adds new WebhookClient and allows you to fetch channel webhooks and such without being "over the top" (#768)

* start blocking out client

* proto webhookclient

* wee working webhooks

* it's all working

* run docs

* fix jsdoc issues

* add example for webhookClient

* add example in the examples place

* fix docs

src/structures/Guild.js

@@ -622,6 +622,14 @@ class Guild {
     return this.client.rest.methods.deleteGuild(this);
   }
 
+  /**
+   * Fetch all webhooks for the guild.
+   * @returns {Collection<Webhook>}
+   */
+  fetchWebhooks() {
+    return this.client.rest.methods.fetchGuildWebhooks(this);
+  }
+
   /**
    * Whether this Guild equals another Guild. It compares all properties, so for most operations
    * it is advisable to just compare `guild.id === guild2.id` as it is much faster and is often