Override the feature filename

Hi @lukaso 

This [one-liner](https://github.com/simplybusiness/rutabaga/compare/override_the_fetaure_filename?w=1#L2L6) lets me write

```
it "implements the feature"
  feature "/path/to/feature/anywhere/I/like.feature"
end
```

which I think looks a lot tidier than putting the feature filename in the `it` clause, especially if you want to `rspec --format doc` or something

The diff probably looks better with [?w=1](https://github.com/simplybusiness/rutabaga/compare/override_the_fetaure_filename?w=1#L2L2) because I took the opportunity to fix up some hard tabs I found at the same time.

I ran the chopin test suite against this branch and it all continues to work.