add channel categories (#1727)

* add channel categories

* add specific class

* speed

* Update Channel.js

* fix type typo

* Update Channel.js

* rewrite position stuff in prep for category sorting

* fix small issues in generation of permissions

* Update Guild.js

* Update Constants.js

* Update GuildChannel.js

* doc fix

* Update GuildChannel.js

* <.<

src/structures/CategoryChannel.js

@@ -0,0 +1,9 @@
+const GuildChannel = require('./GuildChannel');
+
+class CategoryChannel extends GuildChannel {
+  get children() {
+    return this.guild.channels.filter(c => c.parentID === this.id);
+  }
+}
+
+module.exports = CategoryChannel;