api.{Send,Edit}MessageData and their equivalents in package api/webhook
have been updated to add some fields added in Discord API v9.
(webhook.Client).EditMessage now also returns a message, because that
endpoint returns a message on success.
This commit adds subcommand aliases as well as additional code in
HelpGenerate to cover for both subcommand and command aliases.
A breaking change is that {,Must}RegisterSubcommandCustom methods are
now replaced with normal {,Must}RegisterSubcommand methods. This is
because they now use variadic strings, which could take 0, 1 or more
arguments.
This commit also allows AddMiddleware and similar methods to be given a
method directly:
sub.Plumb(cmds.PlumbedHandler)
sub.AddMiddleware(cmds.PlumbedHandler, cmds.plumbMiddleware)
This change closes issue #146.