Feature filename

I may be coming from a similar place as @opsb and request #160.

I am creating a reporter for our PQA team. They organize our feature files into directories; they'd like an HTML report that visualizes test results organized into the same groups. It would be useful to be able to access the source file name given to the runner for each feature from a lifecycle hook (e.g. `after_feature_run`). 

I saw the line attribute on `Spinach::Feature`, but it did not seem to be set. I modded `Spinach::Runner` to set both these values after parsing the feature file sources, but before launching the feature runner.

Please let me know if there's an existing way to get this info; otherwise, this seemed a reasonable way to add the source file name to the Feature instances passed .
Prefer single-quoted strings when you don't need string interpolation or special symbols.