Add promo as set

mlp/card.go

@@ -58,7 +58,7 @@ func LoadCardList(list []string, fetch bool) (out []Card, err error) {
 		for _, promoid := range promoIDs {
 			if strings.ToLower(card) == promoid {
 				card = strings.ToLower(card)
-				setid = "promo"
+				setid = SetPromo
 				break
 			}
 		}

mlp/mlp.go

@@ -30,6 +30,7 @@ const (
 	SetDefendersOfEquestria SetID = "DE"
 	SetSeaquestriaBeyond    SetID = "SB"
 	SetFriendsForever       SetID = "FF"
+	SetPromo                SetID = "Promo"
 )
 
 // BlockID denotes a certain block
@@ -55,4 +56,5 @@ var allSets = []SetID{
 	SetDefendersOfEquestria,
 	SetSeaquestriaBeyond,
 	SetFriendsForever,
+	SetPromo,
 }