feat #19 Flyer - Tri fold?

add trifold option to items

models/printer.go

@@ -15,6 +15,7 @@ const (
 	LongEdge      PrintOption = ""
 	ShortEdge     PrintOption = "-o Binding=TopBinding"
 	CreateBooklet PrintOption = "-o Combination=Booklet -o PageSize=A5"
+	TriFold       PrintOption = "-o Fold=TriFold"
 )
 
 type PrintJob struct {
@@ -32,6 +33,10 @@ func GetPrintMode(mode string) PrintOption {
 		return ShortEdge
 	}
 
+	if mode == "TriFold" {
+		return TriFold
+	}
+
 	return CreateBooklet
 }