Enable redis-cli access to RedisToGo

Accomplish the functionality described in
https://github.com/croaky/parity/issues/8. Using `Open3` from the standard
library to capture the output of the request to Heroku for the URL, which is
then memoized and parsed with regex to get the host, post, and password.

User can then access their RedisToGo with `production redis_cli`.

*PR note:* I couldn't name the `#redis_cli` method with a dash to match the `redis-cli` command, but we could use `rediscli` as the command or automatically map hyphens to underscores from the first argument when passing through to a method if we wanted to keep the exact same command form for `redis-cli`.
What if we keyed off of `REDIS_URL` environment variable instead? There's four or five Redis add-ons with Heroku. It's be nice to work with all of them. Similar technique we use with Suspenders for SMTP variables:

https://github.com/thoughtbot/suspenders/blob/master/templates/smtp.rb

Requires renaming or duplicating the SendGrid or whatever variables but makes things more "standard" and scriptable from libraries like this.

Would require a note in the documentation about the environment variable's interface.