Add support of handling different updates

[glebkin]

Currently, there's no way to handle updates, which does not contain Message or CallbackQuery:

func (b *Bot) ProcessUpdate(ctx context.Context, upd *models.Update) {
	h := b.defaultHandlerFunc

	defer func() {
		applyMiddlewares(h, b.middlewares...)(ctx, b, upd)
	}()

	if upd.Message != nil {
		h = b.findHandler(HandlerTypeMessageText, upd)
		return
	}
	if upd.CallbackQuery != nil {
		h = b.findHandler(HandlerTypeCallbackQueryData, upd)
		return
	}
}

Which is actually a problem, because you can't for example handle channel post updates.

[negasus]

For handle any updates, you can implement your logic in DefaultHandler

	opts := []bot.Option{
		bot.WithDefaultHandler(handler),
	}

	b, err := bot.New(os.Getenv("EXAMPLE_TELEGRAM_BOT_TOKEN"), opts...)

...

func handler(ctx context.Context, b *bot.Bot, update *models.Update) {
	if update.Message != nil {
		// handle message
		return
	}

	// ...

	if update.Poll != nil {
		// handle poll
	}

	// etc...
}

[negasus]

Released in #65