Allow setting URL for redis config

[germsvel]

What changed?

We allow setting a Redis URL in our config via redis_options instead of having to specify a host, port, and password.

E.g.

// instead of this
config :redbird, :redis_options, [host: ...]

// we can now do this
config :redbird, :redis_options, [url: "redis:// ..."]

This is particularly helpful when dealing with Redis + SSL, when the URL scheme changes from redis to rediss.

Implementation details

Redix (the library we use to communicate with Redis) will automatically set ssl: true when it is passed a URL with a rediss scheme.

Unfortunately, the only way to pass a URL to Redix is by passing a modified set of options ({url, list_of_opts} instead of list_of_opts).

Instead of passing those options to Redbird.Redis.start_link/1 only to then pass them again to Redix.start_link/1, we now rely on Redix's child_spec function directly, and put it in our supervision tree.

Finally, to abstract the configuration of options, we create a new module Redbird.Config which handles the merging of options and separating the url from the rest of the options if one is present.

[JacquiManzi]

I'd love to use this project and this is the only blocker for me at the moment.

[relip]

Seems like this PR got approved quite while ago but still not merged into master? As @JacquiManzi mentioned this is the only blocker for us to upgrade redbird to the latest version and we hope to see this to be merged soon.