Added an example of modules using a registry

[etorreborre]

This is a pretty different approach because this is not entirely about modelling effects but more about structuring applications. Since I am often seeing effects being used to structure applications, I think this is an interesting alternative.

Because it might be off-topic feel free to reject this PR!

[stepchowfun]

Hi @etorreborre, thanks for the pull request! I'm not familiar with this approach—is this well-known? A paper or blog post would be helpful for me, if one exists.

Edit: I believe I fixed the CI configuration, so any future commits here will be tested via Travis now.

[etorreborre]

Yes it is described here and there. I will do a presentation on the idea of using a "registry" to wire the modules at Haskell eXchange in October.