Add category support.

This adds:

* CategoryChannel, which represents a category
* Guild.by_category() which traverses the channels grouping by category
* Guild.categories to get a list of categories
* abc.GuildChannel.category to get the category a channel belongs to
* sync_permissions keyword argument to abc.GuildChannel.edit to sync
  permissions with a pre-existing or new category
* category keyword argument to abc.GuildChannel.edit to move a channel
  to a category

docs/api.rst

@@ -1912,6 +1912,13 @@ VoiceChannel
     :members:
     :inherited-members:
 
+CategoryChannel
+~~~~~~~~~~~~~
+
+.. autoclass:: CategoryChannel()
+    :members:
+    :inherited-members:
+
 DMChannel
 ~~~~~~~~~