As this is the work of ~6 weeks effort, there is unfortunately not a
nice commit history, implementing piece-by-piece that can be added.

To ensure that this works correctly, we've utilised feature files from
[gherkin-java], as well as a few custom ones, with a test harness to
ensure that we're able to handle all valid - and invalid - forms of
feature files.

This is bundled in with a test harness, utilising abstract classes to
enable reusability across test cases to validate that:

- we can check whitespace independent - quicker feedback while testing,
  allows making changes without worrying about indentation, and that
  semantically there's nothing lost
- we can check whitespace dependent - the truly important tests

We've gone with method calls in a nested fashion as this lends to easier
selective incrementing of the indentation level. We pass the `State`
around so we can remain stateless, which allows re-using the
`PrettyFormatter` in a thread-safe fashion.

Because we canno...