private fields

[anpic]

@elgopher

Is your feature request related to a problem? Please describe.
please add support private fields too

type A struct {
	A string
	a string
}

m mapify.Mapper
if v, err := m.MapAny(A{ A: "A", a: "a" }); err == nil {
	fmt.Println(v)    // map[A:A]
}

Describe the solution you'd like

m mapify.Mapper
m.PrivateFields = true
m.PublicFields = true
// m.Fields = true // for both
if v, err := m.MapAny(A{ A: "A", a: "a" }); err == nil {
	fmt.Println(v)    // map[A:A,a:a]
}

m mapify.Mapper
m.PrivateFields = true
m.PublicFields = false
if v, err := m.MapAny(A{ A: "A", a: "a" }); err == nil {
	fmt.Println(v)    // map[a:a]
}