func NewMessageItem(ch string, m *Message) Item {
	return Item{Type: TYPE_MESSAGE, Channel: ch, Message: m}
}
NewMessageItem turns a message on a channel into a typed message struct.
package slack
const (
	TYPE_MESSAGE      = "message"
	TYPE_FILE         = "file"
	TYPE_FILE_COMMENT = "file_comment"
	TYPE_CHANNEL      = "channel"
	TYPE_IM           = "im"
	TYPE_GROUP        = "group"
)
type Item struct {
	Type      string   `json:"type"`
	Channel   string   `json:"channel,omitempty"`
	Message   *Message `json:"message,omitempty"`
	File      *File    `json:"file,omitempty"`
	Comment   *Comment `json:"comment,omitempty"`
	Timestamp string   `json:"ts,omitempty"`
}