Add a configuration option for making not-implemented tests failures

[jcornaz]

If there is a mismatch between the *.feature file and the the steps defined, we'll see an output like.

[Cucumber v0.8.4]
Feature: Animal feature                                                                                                                                                                                           features/animal.feature:1:1 

 Scenario: If we feed a hungry cat it will no longer be hungry                                                                                                                                                    features/animal.feature:3:3 
  ✔ Given A hungry cat                                                                                                                                                                                            features/animal.feature:4:5 
  ✔ When I feed the cat                                                                                                                                                                                           features/animal.feature:5:5 
  - Then The cat is not hungry!!!!                                                                                                                                                                                features/animal.feature:6:5 
    ⚡ Not yet implemented (skipped)

[Summary]
1 features
1 scenarios (1 skipped, 0 passed)
6 steps (1 skipped, 2 passed)

And the command line returns 0 (success).

But I don't want to have this to pass the CI and sneak in the main branch.

Ideally if any test was not implemented, the command line should return a failure. Eventually this could be a configurable behavior.

[rob-mur]

From a brief look at the documentation I think we want to change the following code to include skips if specified in the config:

impl Stats {
    /// Indicates this has failing states (aka failed or timed_out)
    pub fn failed(&self) -> bool {
        self.failed > 0 || self.timed_out > 0
    }
}

I'll put it on my list to look at hopefully in the next week or two, but this should be a super simple one to do if anyone has more time!

[tyranron]

Now possible after merging #128

@ilslv we need a meaningful example in the Book for this too.

[tyranron]

Described in docs for Cucumber::fail_on_skipped() and mentioned in "Managing growth" chapter of the Book.