allow to use field name and tag options together

in case when you don't want to duplicate the field name in the
iso8583 tag, you can just skip it and set the rest of the options like
this:

```go
F2 *field.String `iso8583:",keepzero"`
```

then the field id will be derived from the field name and keepzero
option will be used as well

field/index_tag.go

@@ -21,6 +21,18 @@ type IndexTag struct {
 }
 
 func NewIndexTag(field reflect.StructField) IndexTag {
+	indexTag := extractTagInfo(field)
+
+	if indexTag.Tag == "" {
+		id, tag := extractIdAndTagFromName(field.Name)
+		indexTag.ID = id
+		indexTag.Tag = tag
+	}
+
+	return indexTag
+}
+
+func extractTagInfo(field reflect.StructField) IndexTag {
 	// value of the key "index" in the tag
 	var value string
 
@@ -35,41 +47,37 @@ func NewIndexTag(field reflect.StructField) IndexTag {
 	// format of the value is "id[,keep_zero_value]"
 	// id is the id of the field
 	// let's parse it
-	if value != "" {
-		tag, opts := parseTag(value)
-
-		id, err := strconv.Atoi(tag)
-		if err != nil {
-			id = -1
-		}
-
+	if value == "" {
 		return IndexTag{
-			ID:       id,
-			Tag:      tag,
-			KeepZero: opts.Contains("keepzero"),
+			ID: -1,
 		}
 	}
 
-	dataFieldName := field.Name
-	if len(dataFieldName) > 0 && fieldNameIndexRe.MatchString(dataFieldName) {
-		indexStr := dataFieldName[1:]
+	tag, opts := parseTag(value)
+	id, err := strconv.Atoi(tag)
+	if err != nil {
+		id = -1
+	}
+
+	return IndexTag{
+		ID:       id,
+		Tag:      tag,
+		KeepZero: opts.Contains("keepzero"),
+	}
+}
+
+func extractIdAndTagFromName(fieldName string) (int, string) {
+	if len(fieldName) > 0 && fieldNameIndexRe.MatchString(fieldName) {
+		indexStr := fieldName[1:]
 		fieldIndex, err := strconv.Atoi(indexStr)
 		if err != nil {
-			return IndexTag{
-				ID:  -1,
-				Tag: indexStr,
-			}
+			return -1, indexStr
 		}
 
-		return IndexTag{
-			ID:  fieldIndex,
-			Tag: indexStr,
-		}
+		return fieldIndex, indexStr
 	}
 
-	return IndexTag{
-		ID: -1,
-	}
+	return -1, ""
 }
 
 type tagOptions []string
@@ -78,16 +86,13 @@ type tagOptions []string
 // comma-separated options.
 func parseTag(tag string) (string, tagOptions) {
 	tag, opt, _ := strings.Cut(tag, ",")
+
 	return tag, tagOptions(strings.Split(opt, ","))
 }
 
 // Contains reports whether a comma-separated list of options
 // contains a particular substr flag. substr must be surrounded by a
 // string boundary or commas.
 func (o tagOptions) Contains(optionName string) bool {
-	if len(o) == 0 {
-		return false
-	}
-
 	return slices.Contains(o, optionName)
 }