Proposal: Property based testing

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

Elixir aims to merge stream_data into the language in the future. Taking a deeper look into Property Based Testing and 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.