Support connection string for redis cluster

[marswong]

Connection string is commonly used to manage a large scale of redis servers, the api of NewClusterClient is preferred to be:

rdb := redis.NewClusterClient(
    []string{
        "redis://<user>:<pass>@host1:7000/<db>",
        "redis://<user>:<pass>@host2:7000/<db>",
        "redis://<user>:<pass>@host3:7000/<db>"
    }
)

ot just keep compatible with former options:

rdb := redis.NewClusterClient(&redis.ClusterOptions{
    URI: []string{
        "redis://<user>:<pass>@host1:7000/<db>",
        "redis://<user>:<pass>@host2:7000/<db>",
        "redis://<user>:<pass>@host3:7000/<db>"
    },
})

[stephaniehingtgen]

👋 Hey! I just submitted a PR around a new function ParseClusterURLs, like ParseURL but for cluster mode. You could then use that to get the ClusterOptions