Prevent excerpts from being generated by default

:warning: WIP

My attempt at rewriting the tests and modifying Jekyll to not generate a `post.excerpt` value by default.

- [x] Rewrite cucumber tests
- [ ] Rewrite shoulda tests
- [x] Modify core behavior
- [ ] Update documentation

Ref #1860

PS: This is my first time submitting code changes in Ruby or Cucumber, so be gentle. :sweat_smile: