Normalize config materialization

[binarylogic]

Timber's various configuration parameters do not consistently support pulling values via a {:system, "VAR_NAME"} tuple. For example, Timber.Config.api_key/0 does, yet Timber.Config.http_url/0 does not. This makes it challenging to change all configuration URLs via environment variables.

[DavidAntaramian]

We should follow common guidance on library configuration as given in http://michal.muskala.eu/2017/07/30/configuring-elixir-libraries.html#recommendations-for-library-authors

More generically, the configuration should be modifiable at runtime. This allows for users to choose their configuration methodology rather than binding it to a specific type of mechanism. In that regard, it allows users to pull in configuration by their own method (whether that be System.get_env/1 or some other mechanism like etcd or consul).