Add redis-url and redis-insecure-tls command line options

A URL can contain all of the information in the individual flags as a
single string rather than as separate pieces. I've mostly run into URLs
pointing to Redis and this make those existing URLs easier to work with.

This also introduces the -redis-insecure-tls flag which turns off TLS
certificate hostname verification. We've chosen Heroku Redis for a
recent project which requires TLS but, for reasons I don't know, they
don't provide a certificate that is valid for the hostname. I also
wasn't able to get the existing -redis-tls flag to work.

redis_info_handlers.go

@@ -30,7 +30,7 @@ func newRedisInfoHandlerFunc(rdb *redis.Client) http.HandlerFunc {
 		}
 		info := parseRedisInfo(res)
 		resp := RedisInfoResponse{
-			Addr:    flagRedisAddr,
+			Addr:    rdb.Options().Addr,
 			Info:    info,
 			RawInfo: res,
 		}