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