Command system overhaul

[Corentin-Luc-Artaud]

• Transform commands into objects respecting the following interface:

type Command interface {
    Check() boolean
    Help() string
    Action(b *Bot, ...)
    Fallback(b *Bot, ...)
}

• help command lists all available commands along a short explicative message about the bot
• help cmdName returns the specific help for the given command
• Every command can take help as an argument, which returns the commands Help() containing the help message
• When a bad arg is given, the command returns its Help()