Add support for parse sentinel url

[chlins]

Currently, go-redis provides a helper function ParseURL which can parse redis url to options, but it not work when we use sentinel mode, and here is a background we define the sentinel url like redis+sentinel://anonymous:password@host1:26379,host2:26379/mymaster/1?idle_timeout_seconds=30, so i propose add a function to parse this way or expose the inner helper function to public, then we could import and only need to implement a simple adapter code in our project. link: https://github.com/go-redis/redis/blob/d28f1f8d391a77adf88f1980e5926c5131eb4ff8/options.go#L222