create redis store with a username and password

[sunpengfei0307]

Hi! I check the api:
"github.com/gin-contrib/sessions/redis"

1. func NewStoreWithDB(size int, network, address, password, DB string, keyPairs ...[]byte) (Store, error), no username filed,

2. can't reuse the go-redis's handle (created by redis.NewClient())，I have to create a redundant redisgo‘s connection。