func (s *Slack) AddReceivers(channelIDs ...string) {
	s.channelIDs = append(s.channelIDs, channelIDs...)
}
AddReceivers takes Slack channel IDs and adds them to the internal channel ID list. The Send method will send
a given message to all those channels.
package slack
import (
	"context"
	"github.com/pkg/errors"
	"github.com/slack-go/slack"
)
type Slack struct {
	client     *slack.Client
	channelIDs []string
}
func New(apiToken string) *Slack {
	client := slack.New(apiToken)
	s := &Slack{
		client:     client,
		channelIDs: []string{},
	}
	return s
}