Ballet

* Support using holdout data splits in validation ([75](https://github.com/ballet/ballet/pull/75))
* Fix CLI program name in projects ([74](https://github.com/ballet/ballet/pull/74))
* Fix bug with `load_config` usage in python REPL ([73](https://github.com/ballet/ballet/pull/73))
* Reorganize external feature engineering primitives to `ballet/eng/external/**.py`. Imports like `from ballet.eng.external import MyPrimitive` are unaffected.

* Fix upgrade check in `ballet update-project-template` to migrate away from deprecated PyPI XML-RPC API.

* Fix links in project template

* Automate creation of GitHub repository in quickstart

* Allow validation to be run from topic branches locally

* Add `Project.version` property