Add initial transmitter implementation (discord)

This has been tested with one webhook in one channel.

Sends, edits and deletions work fine

bridge/discord/handlers.go

@@ -69,7 +69,7 @@ func (b *Bdiscord) messageCreate(s *discordgo.Session, m *discordgo.MessageCreat
 		return
 	}
 	// if using webhooks, do not relay if it's ours
-	if b.useWebhook() && m.Author.Bot && b.isWebhookID(m.Author.ID) {
+	if m.Author.Bot && b.transmitter.HasWebhook(m.Author.ID) {
 		return
 	}