feat: Support custom file name mappings

[driventokill]

Custom field name mappings to copy values with different names in fromValue and toValue types. Examples can be found in copier_different_field_name_test.go.

resolve #139

type User1 struct {
	Id      int64
	Name    string
	Address []string
}

type User2 struct {
	Id2      int64
	Name2    string
	Address2 []string
}

u1 := User1{Id: 1, Name: "1", Address: []string{"1"}}
var u2 User2
err := copier.CopyWithOption(&u2, u1, copier.Option{FieldNameMapping: []copier.FieldNameMapping{
	{SrcType: u1, DstType: u2,
		Mapping: map[string]string{
			"Id":      "Id2",
			"Name":    "Name2",
			"Address": "Address2"}},
}})

[driventokill]

@jinzhu The failed cases seem like been broken since f036a42 (Merge pull request #178 from QianChenglong/master)