Chore embed crummy-test in crummy

Hi @zachinglis ! Yesterday, taking a look at Crummy I read what you had written in the Crummy-test about not having found a satisfactory way to test your gem _from inside_. I also read some posts you wrote about learning and teaching, and I liked them.

A few weeks ago, I saw an example app embedded into a gem ([TBK][tbk]) and I found that quite clean. I liked the idea, some aspects of some gems just can't be tested without an app. The example app becomes part of the documentation or part of the test suite and can be maintained as so. Debugging also becomes a lot less painful without the need to bump versions for any change.

  [tbk]: https://github.com/sagmor/tbk

If you hadn't seen this done before, here is an example, I hope it'll help you.