Add support for Umbrella-app scoped configuration

[feda12]

Currently, it is not possible to define multiple configurations in an umbrella app because the configuration variables are global.

This addresses this by attempting to find an app-specific config object first, then defaulting to a global one. This ensures backward compatibility.

[ghost]

👍
I have only one phoenix app in my umbrella and having an issue because of the https://github.com/KittyHeaven/blue_bird/blob/master/lib/blue_bird/generator.ex#L88

My umbrella will run the different test and the last one that runs is not the phoenix app. So Project.get().application will not return the right app, if this app doesn't have a :mod it will crash.

Seems like this PR is fixing my issue because I am happy with the default configuration.

But I think you will have unpredictable results with

    Project.get().project()
    |> Keyword.get(:app)
    |> Application.get_env(:blue_bird, nil)

because of the order of the tests.

[feda12]

Sorry for the delay, been attempting to work around it. Will update when I have something solid.

[feda12]

I made more changes to accommodate our own needs. If you think these make sense, happy to merge them otherwise I am going to close this for now.