This commit changes the existing Plumb behavior to allow normal commands
to coexist along a plumbed command. This change allows certain behaviors
that would otherwise require manually switching on arguments.
An example use case of this change would be having a default behavior
when a subcommand call doesn't have a command name. For example, given
this code:
func (b *Banana) Setup(sub *bot.Subcommand) { sub.SetPlumb(b.Help) }
func (b *Banana) Green(*gateway.MessageCreateEvent) {}
func (b *Banana) Help(*gateway.MessageCreateEvent) {}
The subcommand "banana" could have its own help when it's called as
"!banana", while "!banana green" would trigger another handler.