Shabadoo

- Fixes 14 - A bug in which the formula printer did not include plus signs.
- Fixes 11 - Allow users to set the rng seed when randomness is used (`.fit`, `.sample_posterior_predictive`). Thanks favianrahman for the suggestion!
- Fixes 6 - Add a confidence interval option in the `.predict` method.
- Add this changelog!

- Init releases. I made many because there were some problems with the pypi auto release.