Yield scenario status to blocks during execution

See https://github.com/cucumber/cucumber/issues/450

People would like to be able to know the state of the scenario in a block, the classic case being:

```
After { |scenario| save_and_open_page if scenario.failed? }
```

Right now we can't support this, as the hooks don't get any information about the passing / failing state. We should figure out a mechanism to do that.