added converter option for type conversions

[tomdevelops]

You can add now converter functions as an option, to convert type A to to type B.

I took some inspiration from PR #90, but implemented it a little bit diffrent. Some of the diffrences are as follows:

• passed it as an option
• no instances
• no exposure of reflect package
• no external module dependencies

Example

func main() {
	type SrcStruct struct {
		Field1 time.Time
		Field2 *time.Time
	}

	type DestStruct struct {
		Field1 string
		Field2 string
	}

	testTime := time.Date(2021, 3, 5, 1, 30, 0, 123000000, time.UTC)

	src := SrcStruct{
		Field1: testTime,
		Field2: &testTime,
	}

	var dst DestStruct

	err := copier.CopyWithOption(&dst, &src, copier.Option{
		IgnoreEmpty: true,
		DeepCopy:    true,
		Converters: []copier.TypeConverter{
			{
				SrcType: time.Time{},
				DstType: copier.String,
				Fn: func(src interface{}) (interface{}, error) {
					s, ok := src.(time.Time)

					if !ok {
						return nil, errors.New("src type not matching")
					}

					return s.Format(time.RFC3339), nil
				},
			},
		},
	})

	if err != nil {
		log.Fatal(err)
	}
	
	// DestStruct{Field1:"2021-03-05T01:30:00Z", Field2:"2021-03-05T01:30:00Z"}
	fmt.Printf("%#v", dst)
}

Feel free to make suggestions to any part of the PR.