Add initial transmitter implementation (discord)

This has been tested with one webhook in one channel.

Sends, edits and deletions work fine

bridge/discord/transmitter/utils.go

@@ -0,0 +1,32 @@
+package transmitter
+
+import (
+	"github.com/matterbridge/discordgo"
+)
+
+// isDiscordPermissionError returns false for nil, and true if a Discord RESTError with code discordgo.ErrorCodeMissionPermissions
+func isDiscordPermissionError(err error) bool {
+	if err == nil {
+		return false
+	}
+
+	restErr, ok := err.(*discordgo.RESTError)
+	if !ok {
+		return false
+	}
+
+	return restErr.Message != nil && restErr.Message.Code == discordgo.ErrCodeMissingPermissions
+}
+
+// getDiscordUserID gets own user ID from state, and fallback on API request
+func getDiscordUserID(session *discordgo.Session) (string, error) {
+	if user := session.State.User; user != nil {
+		return user.ID, nil
+	}
+
+	user, err := session.User("@me")
+	if err != nil {
+		return "", err
+	}
+	return user.ID, nil
+}