Add `Then` block (#49)

```
Given (Some text):
  Something

Do (Convert it to lowercase and yank the word):
  ye
  guu

Then (@" register should hold the original word):
  AssertEqual 'Something', @"

Expect (But the buffer should contain text in lowercase):
  something
```

Can be used to replace hacky use of After blocks;
e.g. https://github.com/junegunn/vim-oblique/blob/9d11d2/test/oblique.vader#L6-L12