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.
