Allow runtime delegator configuration

[tlux]

We should allow to change the delegator at runtime as for some test scenarios this may make sense.

Application.put_env(:my_app, Delx, delegator: MyCustomDelegator)

API Suggestion:

defmodule MyModule do
  use Delx, otp_app: :my_app, runtime: true
end

Compile time delegation (runtime: false) should be the default though as runtime delegation is significantly slower.