[Sentinel] Missing manual failover command

[EtienneM]

Redis documentation lists the SENTINEL commands (https://redis.io/topics/sentinel#sentinel-commands). I would love to be able to run SENTINEL failover <master name> using your library. Would you agree if I make a PR which basically adds the following method?

func (c *SentinelClient) Failover(name string) *StatusCmd {
	cmd := NewStatusCmd("sentinel", "failover", name)
	c.Process(cmd)
	return cmd
}

[vmihailenco]

Yes, sounds good