Copy different field name from tag flag

[tomtwinkle]

Specifying a name for the copier:"<Name>" tag, it is possible to copy the value to a field with a different field name.
To distinguish it from other flags, only the public field is supported.

	type SrcTags struct {
		FieldA string
		FieldB string `copier:"Field2"`
		FieldC string `copier:"FieldTagMatch"`
	}

	type DestTags struct {
		Field1 string `copier:"FieldA"`
		Field2 string
		Field3 string `copier:"FieldTagMatch"`
	}

	dst := &DestTags{}
	src := &SrcTags{
		FieldA: "FieldA->Field1",
		FieldB: "FieldB->Field2",
		FieldC: "FieldC->Field3",
	}
	copier.Copy(dst, src)

	fmt.Printf("%+v", dst)
	// &{Field1:FieldA->Field1 Field2:FieldB->Field2 Field3:FieldC->Field3}

[jinzhu]

thank you for your PR.