Add Guild.defaultChannel. (#646)

* Add Guild.defaultChannel. Closes #643.

* Change defaultChannel to a getter.

src/structures/Guild.js

@@ -226,6 +226,15 @@ class Guild {
     return this.members.get(this.ownerID);
   }
 
+  /**
+   * The `#general` GuildChannel of the server.
+   * @type {GuildChannel}
+   * @readonly
+   */
+  get defaultChannel() {
+    return this.channels.get(this.id);
+  }
+
   /**
    * Updates the Guild with new information - e.g. a new name.
    * @param {GuildEditData} data The data to update the guild with