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/TextChannel.js

@@ -57,6 +57,9 @@ class TextChannel extends GuildChannel {
   createCollector() { return; }
   awaitMessages() { return; }
   bulkDelete() { return; }
+  fetchWebhook() { return; }
+  fetchWebhooks() { return; }
+  createWebhook() { return; }
   _cacheMessage() { return; }
 }