proposal: support mapping between pointer and value

I plan to make a new feature to support mapping between pointer and value

example:

```go
type A struct{
    Age int `mapper:"age"`
}

type B struct{
    Age *int `mapper:"age"`
} 

func main(){
    a := A{Age:1}
    b := B{}
    m := mapper.NewMapper()

    m.Mapper(&a, &b)

    // now b.Age = 1
}
```