Added support for running Cucumber .feature files.

* This works with any `.feature` file, however, if the feature file is not in the base level directory then cucumber wont run because it can't find the step definitions. I'm hoping to fix this but I think it may be an issue with Atom's `BufferedProcess`

* The if statement on line 65 could probably be improved. Hoping to write some tests in the future and then refactor.