code: func NewMapperTagFunc(tagName string, mapFunc, tagMapFunc func(string) string) *Mapper {
  	return &Mapper{
  		cache:      make(map[reflect.Type]*StructMap),
  		tagName:    tagName,
  		mapFunc:    mapFunc,
  		tagMapFunc: tagMapFunc,
  	}
  }
docs: // NewMapperTagFunc returns a new mapper which contains a mapper for field names
  // AND a mapper for tag values.  This is useful for tags like json which can
  // have values like "name,omitempty".