Allow to provide icon and or icon_emoji

[Pavel Kaluhin]

I found that you allow to provide icon or icon_emoji in values

type slackPayload struct {
	Channel     string            `json:"channel"`
	Username    string            `json:"username"`
	IconUrl     string            `json:"icon_url"`
	IconEmoji   string            `json:"icon_emoji"`
	Text        string            `json:"text,omitempty"`
	Attachments []slackAttachment `json:"attachments,omitempty"`
}

But it's not actually used

payload := slackPayload{
		Channel:  cfg.Channel,
		Username: cfg.Username,
	}

Could you add it?