Support "AddFlat" feature for struct

[momotaro98]

When we have the followings

	type Post struct {
		Title  string `redis:"title"`
		UserID int64  `redis:"user_id"`
	}

	post := Post{
		Title:  "First Game",
		UserID: 123,
	}

redigo supports converting redis tag fields of a struct to binary like below.

	import (
		redigo "github.com/gomodule/redigo/redis"
	)

	 _, err := conn.Do("HSET", redis.Args{}.Add(key).AddFlat(&post)...)

This above works.

The code below is go-redis case.

	import (
		"github.com/go-redis/redis/v8"
	)

	_, err := c.cli.HSet(context.Background(), key, &post).Result()

I know the err says redis: can't marshal *main.Post (implement encoding.BinaryMarshaler) so I should implement encoding.BinaryMarshaler for Post struct but I believe it's good if go-redis also supports the AddFlat feature even though it can be a bit more complex than current one as well as redigo.