Support for omit empty in tag

``` go
type Example struct {
    Field1 string `json:"field1,omitempty"`
    Field2 int    `json:"field2,omitempty"`
    Field3 bool   `json:"field3,omitempty"`
    Field4 *int   `json:"field4,omitempty"`
}
```

JSon have this `omitempty` to omit a field if it empty. is there anyway to do similar thing in this library boss @shamaton  ?