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.