Add Config.read_config/1

[josevalim]

No description provided.

[josevalim]

A potential question we could have here is if we should mirror the functionality in Application. If we were to follow their API, we would have:

• read_all_config(root_key)
• read_config(root_key, path)

The reason I chose only a single read_config/1 is to give more control to the user. Although the path one can be a bit more convenient, I don't foresee many use cases anyway. I want to make it explicitly distinct from the application one (as to avoid any confusion that they are related) and also because they have different performance implications. get_all_env, for example, performs multiple ETS reads.

If we want, in the future, we could add: read_config(root_key, path) and bang versions for both.

[sabiwara]

LGTM

[josevalim]

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