Add a few BDD/TDD line based executions

I've updated "Selection Based" support to pass a [`Selection`](https://atom.io/docs/api/v0.89.0/api/classes/Selection.html) instead of the selection text. The selection block now takes `filename` and `selection` instead. To get the selected text now `selection.getText()` is used.

- [x] Cucumber Gherkin line based execution support
- [x] Behat Gherkin line based execution support
- [ ] RSpec line based execution vs. regular selection based execution support

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![gherkin_line_support](https://cloud.githubusercontent.com/assets/1694055/2747658/5f493680-c77b-11e3-8dcf-a0cb2152901a.gif)