Fixes defaults not being overriden by config

[victorsolis]

This also adds an error when Authsense is not configured at compile time. It was previously raising a no match error at runtime when it wasn't configured.

[rstacruz]

btw:

database_url = System.get_env("DATABASE_URL") || "postgres://postgres:postgres@localhost/authsense_test"

config :authsense, Authsense.Test.Repo,
  url: database_url

[rstacruz]

...but that maybe for another PR, i dunno

[victorsolis]

As per thy bidding

** (Authsense.MultipleResourcesException) Multiple resources are configured.

    Authsense cannot determine which resource module to use.
    Available resource modules: [Admin, User]

    lib/authsense.ex:138: Authsense.config/1

[rstacruz]

when is the dev environment ever used in a Hex package like this?

[rstacruz]

The existence of config/dev.exs is very strange to me (it's not like Authsense has a development mode), but lemme merge and release this nonetheless. Great work. Reviewed and merged. Thank you for your contribution. 💚

[victorsolis]

config/dev.exs is just there so you don't get an exception when you run iex -S mix.

I am humbled by your appreciation.

[rstacruz]

Ah, right, carry on