Proposal: Property based testing

[Elixir plans to include Property Based Testing in the future.][elixir-pbt] As such it would make sense to think about how to include it into ESpec.

Elixir aims to merge [`stream_data`][stream_data] into the language in the future. Taking a deeper look into Property Based Testing and [`stream_data`][stream_data] would prepare us for integrating it into ESpec. This issue should serve as a platform for discussion on how this can be achieved while keeping the spec-based approach which makes ESpec such a pleasure to use.

[elixir-pbt]: https://elixirforum.com/t/questions-about-property-testing-stream-data/9445
[stream_data]: https://github.com/whatyouhide/stream_data